linux/Documentation/trace/ftrace.rst

========================
ftrace - Function Tracer
========================

Copyright 2008 Red Hat Inc.

:Author:   Steven Rostedt <srostedt@redhat.com>
:License:  The GNU Free Documentation License, Version 1.2
          (dual licensed under the GPL v2)
:Original Reviewers:  Elias Oltmanns, Randy Dunlap, Andrew Morton,
		      John Kacur, and David Teigland.

- Written for: 2.6.28-rc2
- Updated for: 3.10
- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
- Converted to rst format - Changbin Du <changbin.du@intel.com>

Introduction
------------

Ftrace is an internal tracer designed to help out developers and
designers of systems to find what is going on inside the kernel.
It can be used for debugging or analyzing latencies and
performance issues that take place outside of user-space.

Although ftrace is typically considered the function tracer, it
is really a framework of several assorted tracing utilities.
There's latency tracing to examine what occurs between interrupts
disabled and enabled, as well as for preemption and from a time
a task is woken to the task is actually scheduled in.

One of the most common uses of ftrace is the event tracing.
Throughout the kernel is hundreds of static event points that
can be enabled via the tracefs file system to see what is
going on in certain parts of the kernel.

See events.rst for more information.


Implementation Details
----------------------

See Documentation/trace/ftrace-design.rst for details for arch porters and such.


The File System
---------------

Ftrace uses the tracefs file system to hold the control files as
well as the files to display output.

When tracefs is configured into the kernel (which selecting any ftrace
option will do) the directory /sys/kernel/tracing will be created. To mount
this directory, you can add to your /etc/fstab file::

 tracefs       /sys/kernel/tracing       tracefs defaults        0       0

Or you can mount it at run time with::

 mount -t tracefs nodev /sys/kernel/tracing

For quicker access to that directory you may want to make a soft link to
it::

 ln -s /sys/kernel/tracing /tracing

.. attention::

  Before 4.1, all ftrace tracing control files were within the debugfs
  file system, which is typically located at /sys/kernel/debug/tracing.
  For backward compatibility, when mounting the debugfs file system,
  the tracefs file system will be automatically mounted at:

  /sys/kernel/debug/tracing

  All files located in the tracefs file system will be located in that
  debugfs file system directory as well.

.. attention::

  Any selected ftrace option will also create the tracefs file system.
  The rest of the document will assume that you are in the ftrace directory
  (cd /sys/kernel/tracing) and will only concentrate on the files within that
  directory and not distract from the content with the extended
  "/sys/kernel/tracing" path name.

That's it! (assuming that you have ftrace configured into your kernel)

After mounting tracefs you will have access to the control and output files
of ftrace. Here is a list of some of the key files:


 Note: all time values are in microseconds.

  current_tracer:

	This is used to set or display the current tracer
	that is configured. Changing the current tracer clears
	the ring buffer content as well as the "snapshot" buffer.

  available_tracers:

	This holds the different types of tracers that
	have been compiled into the kernel. The
	tracers listed here can be configured by
	echoing their name into current_tracer.

  tracing_on:

	This sets or displays whether writing to the trace
	ring buffer is enabled. Echo 0 into this file to disable
	the tracer or 1 to enable it. Note, this only disables
	writing to the ring buffer, the tracing overhead may
	still be occurring.

	The kernel function tracing_off() can be used within the
	kernel to disable writing to the ring buffer, which will
	set this file to "0". User space can re-enable tracing by
	echoing "1" into the file.

	Note, the function and event trigger "traceoff" will also
	set this file to zero and stop tracing. Which can also
	be re-enabled by user space using this file.

  trace:

	This file holds the output of the trace in a human
	readable format (described below). Opening this file for
	writing with the O_TRUNC flag clears the ring buffer content.
        Note, this file is not a consumer. If tracing is off
        (no tracer running, or tracing_on is zero), it will produce
        the same output each time it is read. When tracing is on,
        it may produce inconsistent results as it tries to read
        the entire buffer without consuming it.

  trace_pipe:

	The output is the same as the "trace" file but this
	file is meant to be streamed with live tracing.
	Reads from this file will block until new data is
	retrieved.  Unlike the "trace" file, this file is a
	consumer. This means reading from this file causes
	sequential reads to display more current data. Once
	data is read from this file, it is consumed, and
	will not be read again with a sequential read. The
	"trace" file is static, and if the tracer is not
	adding more data, it will display the same
	information every time it is read.

  trace_options:

	This file lets the user control the amount of data
	that is displayed in one of the above output
	files. Options also exist to modify how a tracer
	or events work (stack traces, timestamps, etc).

  options:

	This is a directory that has a file for every available
	trace option (also in trace_options). Options may also be set
	or cleared by writing a "1" or "0" respectively into the
	corresponding file with the option name.

  tracing_max_latency:

	Some of the tracers record the max latency.
	For example, the maximum time that interrupts are disabled.
	The maximum time is saved in this file. The max trace will also be
	stored,	and displayed by "trace". A new max trace will only be
	recorded if the latency is greater than the value in this file
	(in microseconds).

	By echoing in a time into this file, no latency will be recorded
	unless it is greater than the time in this file.

  tracing_thresh:

	Some latency tracers will record a trace whenever the
	latency is greater than the number in this file.
	Only active when the file contains a number greater than 0.
	(in microseconds)

  buffer_percent:

	This is the watermark for how much the ring buffer needs to be filled
	before a waiter is woken up. That is, if an application calls a
	blocking read syscall on one of the per_cpu trace_pipe_raw files, it
	will block until the given amount of data specified by buffer_percent
	is in the ring buffer before it wakes the reader up. This also
	controls how the splice system calls are blocked on this file::

	  0   - means to wake up as soon as there is any data in the ring buffer.
	  50  - means to wake up when roughly half of the ring buffer sub-buffers
	        are full.
	  100 - means to block until the ring buffer is totally full and is
	        about to start overwriting the older data.

  buffer_size_kb:

	This sets or displays the number of kilobytes each CPU
	buffer holds. By default, the trace buffers are the same size
	for each CPU. The displayed number is the size of the
	CPU buffer and not total size of all buffers. The
	trace buffers are allocated in pages (blocks of memory
	that the kernel uses for allocation, usually 4 KB in size).
	A few extra pages may be allocated to accommodate buffer management
	meta-data. If the last page allocated has room for more bytes
	than requested, the rest of the page will be used,
	making the actual allocation bigger than requested or shown.
	( Note, the size may not be a multiple of the page size
	due to buffer management meta-data. )

	Buffer sizes for individual CPUs may vary
	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
	this file will show "X".

  buffer_total_size_kb:

	This displays the total combined size of all the trace buffers.

  free_buffer:

	If a process is performing tracing, and the ring buffer	should be
	shrunk "freed" when the process is finished, even if it were to be
	killed by a signal, this file can be used for that purpose. On close
	of this file, the ring buffer will be resized to its minimum size.
	Having a process that is tracing also open this file, when the process
	exits its file descriptor for this file will be closed, and in doing so,
	the ring buffer will be "freed".

	It may also stop tracing if disable_on_free option is set.

  tracing_cpumask:

	This is a mask that lets the user only trace on specified CPUs.
	The format is a hex string representing the CPUs.

  set_ftrace_filter:

	When dynamic ftrace is configured in (see the
	section below "dynamic ftrace"), the code is dynamically
	modified (code text rewrite) to disable calling of the
	function profiler (mcount). This lets tracing be configured
	in with practically no overhead in performance.  This also
	has a side effect of enabling or disabling specific functions
	to be traced. Echoing names of functions into this file
	will limit the trace to only those functions.
	This influences the tracers "function" and "function_graph"
	and thus also function profiling (see "function_profile_enabled").

	The functions listed in "available_filter_functions" are what
	can be written into this file.

	This interface also allows for commands to be used. See the
	"Filter commands" section for more details.

	As a speed up, since processing strings can be quite expensive
	and requires a check of all functions registered to tracing, instead
	an index can be written into this file. A number (starting with "1")
	written will instead select the same corresponding at the line position
	of the "available_filter_functions" file.

  set_ftrace_notrace:

	This has an effect opposite to that of
	set_ftrace_filter. Any function that is added here will not
	be traced. If a function exists in both set_ftrace_filter
	and set_ftrace_notrace,	the function will _not_ be traced.

  set_ftrace_pid:

	Have the function tracer only trace the threads whose PID are
	listed in this file.

	If the "function-fork" option is set, then when a task whose
	PID is listed in this file forks, the child's PID will
	automatically be added to this file, and the child will be
	traced by the function tracer as well. This option will also
	cause PIDs of tasks that exit to be removed from the file.

  set_ftrace_notrace_pid:

        Have the function tracer ignore threads whose PID are listed in
        this file.

        If the "function-fork" option is set, then when a task whose
	PID is listed in this file forks, the child's PID will
	automatically be added to this file, and the child will not be
	traced by the function tracer as well. This option will also
	cause PIDs of tasks that exit to be removed from the file.

        If a PID is in both this file and "set_ftrace_pid", then this
        file takes precedence, and the thread will not be traced.

  set_event_pid:

	Have the events only trace a task with a PID listed in this file.
	Note, sched_switch and sched_wake_up will also trace events
	listed in this file.

	To have the PIDs of children of tasks with their PID in this file
	added on fork, enable the "event-fork" option. That option will also
	cause the PIDs of tasks to be removed from this file when the task
	exits.

  set_event_notrace_pid:

	Have the events not trace a task with a PID listed in this file.
	Note, sched_switch and sched_wakeup will trace threads not listed
	in this file, even if a thread's PID is in the file if the
        sched_switch or sched_wakeup events also trace a thread that should
        be traced.

	To have the PIDs of children of tasks with their PID in this file
	added on fork, enable the "event-fork" option. That option will also
	cause the PIDs of tasks to be removed from this file when the task
	exits.

  set_graph_function:

	Functions listed in this file will cause the function graph
	tracer to only trace these functions and the functions that
	they call. (See the section "dynamic ftrace" for more details).
	Note, set_ftrace_filter and set_ftrace_notrace still affects
	what functions are being traced.

  set_graph_notrace:

	Similar to set_graph_function, but will disable function graph
	tracing when the function is hit until it exits the function.
	This makes it possible to ignore tracing functions that are called
	by a specific function.

  available_filter_functions:

	This lists the functions that ftrace has processed and can trace.
	These are the function names that you can pass to
	"set_ftrace_filter", "set_ftrace_notrace",
	"set_graph_function", or "set_graph_notrace".
	(See the section "dynamic ftrace" below for more details.)

  available_filter_functions_addrs:

	Similar to available_filter_functions, but with address displayed
	for each function. The displayed address is the patch-site address
	and can differ from /proc/kallsyms address.

  dyn_ftrace_total_info:

	This file is for debugging purposes. The number of functions that
	have been converted to nops and are available to be traced.

  enabled_functions:

	This file is more for debugging ftrace, but can also be useful
	in seeing if any function has a callback attached to it.
	Not only does the trace infrastructure use ftrace function
	trace utility, but other subsystems might too. This file
	displays all functions that have a callback attached to them
	as well as the number of callbacks that have been attached.
	Note, a callback may also call multiple functions which will
	not be listed in this count.

	If the callback registered to be traced by a function with
	the "save regs" attribute (thus even more overhead), a 'R'
	will be displayed on the same line as the function that
	is returning registers.

	If the callback registered to be traced by a function with
	the "ip modify" attribute (thus the regs->ip can be changed),
	an 'I' will be displayed on the same line as the function that
	can be overridden.

	If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
	Note, normal ftrace trampolines can also be attached, but only one
	"direct" trampoline can be attached to a given function at a time.

	Some architectures can not call direct trampolines, but instead have
	the ftrace ops function located above the function entry point. In
	such cases an 'O' will be displayed.

	If a function had either the "ip modify" or a "direct" call attached to
	it in the past, a 'M' will be shown. This flag is never cleared. It is
	used to know if a function was every modified by the ftrace infrastructure,
	and can be used for debugging.

	If the architecture supports it, it will also show what callback
	is being directly called by the function. If the count is greater
	than 1 it most likely will be ftrace_ops_list_func().

	If the callback of a function jumps to a trampoline that is
	specific to the callback and which is not the standard trampoline,
	its address will be printed as well as the function that the
	trampoline calls.

  touched_functions:

	This file contains all the functions that ever had a function callback
	to it via the ftrace infrastructure. It has the same format as
	enabled_functions but shows all functions that have every been
	traced.

	To see any function that has every been modified by "ip modify" or a
	direct trampoline, one can perform the following command:

	grep ' M ' /sys/kernel/tracing/touched_functions

  function_profile_enabled:

	When set it will enable all functions with either the function
	tracer, or if configured, the function graph tracer. It will
	keep a histogram of the number of functions that were called
	and if the function graph tracer was configured, it will also keep
	track of the time spent in those functions. The histogram
	content can be displayed in the files:

	trace_stat/function<cpu> ( function0, function1, etc).

  trace_stat:

	A directory that holds different tracing stats.

  kprobe_events:

	Enable dynamic trace points. See kprobetrace.rst.

  kprobe_profile:

	Dynamic trace points stats. See kprobetrace.rst.

  max_graph_depth:

	Used with the function graph tracer. This is the max depth
	it will trace into a function. Setting this to a value of
	one will show only the first kernel function that is called
	from user space.

  printk_formats:

	This is for tools that read the raw format files. If an event in
	the ring buffer references a string, only a pointer to the string
	is recorded into the buffer and not the string itself. This prevents
	tools from knowing what that string was. This file displays the string
	and address for	the string allowing tools to map the pointers to what
	the strings were.

  saved_cmdlines:

	Only the pid of the task is recorded in a trace event unless
	the event specifically saves the task comm as well. Ftrace
	makes a cache of pid mappings to comms to try to display
	comms for events. If a pid for a comm is not listed, then
	"<...>" is displayed in the output.

	If the option "record-cmd" is set to "0", then comms of tasks
	will not be saved during recording. By default, it is enabled.

  saved_cmdlines_size:

	By default, 128 comms are saved (see "saved_cmdlines" above). To
	increase or decrease the amount of comms that are cached, echo
	the number of comms to cache into this file.

  saved_tgids:

	If the option "record-tgid" is set, on each scheduling context switch
	the Task Group ID of a task is saved in a table mapping the PID of
	the thread to its TGID. By default, the "record-tgid" option is
	disabled.

  snapshot:

	This displays the "snapshot" buffer and also lets the user
	take a snapshot of the current running trace.
	See the "Snapshot" section below for more details.

  stack_max_size:

	When the stack tracer is activated, this will display the
	maximum stack size it has encountered.
	See the "Stack Trace" section below.

  stack_trace:

	This displays the stack back trace of the largest stack
	that was encountered when the stack tracer is activated.
	See the "Stack Trace" section below.

  stack_trace_filter:

	This is similar to "set_ftrace_filter" but it limits what
	functions the stack tracer will check.

  trace_clock:

	Whenever an event is recorded into the ring buffer, a
	"timestamp" is added. This stamp comes from a specified
	clock. By default, ftrace uses the "local" clock. This
	clock is very fast and strictly per cpu, but on some
	systems it may not be monotonic with respect to other
	CPUs. In other words, the local clocks may not be in sync
	with local clocks on other CPUs.

	Usual clocks for tracing::

	  # cat trace_clock
	  [local] global counter x86-tsc

	The clock with the square brackets around it is the one in effect.

	local:
		Default clock, but may not be in sync across CPUs

	global:
		This clock is in sync with all CPUs but may
		be a bit slower than the local clock.

	counter:
		This is not a clock at all, but literally an atomic
		counter. It counts up one by one, but is in sync
		with all CPUs. This is useful when you need to
		know exactly the order events occurred with respect to
		each other on different CPUs.

	uptime:
		This uses the jiffies counter and the time stamp
		is relative to the time since boot up.

	perf:
		This makes ftrace use the same clock that perf uses.
		Eventually perf will be able to read ftrace buffers
		and this will help out in interleaving the data.

	x86-tsc:
		Architectures may define their own clocks. For
		example, x86 uses its own TSC cycle clock here.

	ppc-tb:
		This uses the powerpc timebase register value.
		This is in sync across CPUs and can also be used
		to correlate events across hypervisor/guest if
		tb_offset is known.

	mono:
		This uses the fast monotonic clock (CLOCK_MONOTONIC)
		which is monotonic and is subject to NTP rate adjustments.

	mono_raw:
		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
		which is monotonic but is not subject to any rate adjustments
		and ticks at the same rate as the hardware clocksource.

	boot:
		This is the boot clock (CLOCK_BOOTTIME) and is based on the
		fast monotonic clock, but also accounts for time spent in
		suspend. Since the clock access is designed for use in
		tracing in the suspend path, some side effects are possible
		if clock is accessed after the suspend time is accounted before
		the fast mono clock is updated. In this case, the clock update
		appears to happen slightly sooner than it normally would have.
		Also on 32-bit systems, it's possible that the 64-bit boot offset
		sees a partial update. These effects are rare and post
		processing should be able to handle them. See comments in the
		ktime_get_boot_fast_ns() function for more information.

	tai:
		This is the tai clock (CLOCK_TAI) and is derived from the wall-
		clock time. However, this clock does not experience
		discontinuities and backwards jumps caused by NTP inserting leap
		seconds. Since the clock access is designed for use in tracing,
		side effects are possible. The clock access may yield wrong
		readouts in case the internal TAI offset is updated e.g., caused
		by setting the system time or using adjtimex() with an offset.
		These effects are rare and post processing should be able to
		handle them. See comments in the ktime_get_tai_fast_ns()
		function for more information.

	To set a clock, simply echo the clock name into this file::

	  # echo global > trace_clock

	Setting a clock clears the ring buffer content as well as the
	"snapshot" buffer.

  trace_marker:

	This is a very useful file for synchronizing user space
	with events happening in the kernel. Writing strings into
	this file will be written into the ftrace buffer.

	It is useful in applications to open this file at the start
	of the application and just reference the file descriptor
	for the file::

		void trace_write(const char *fmt, ...)
		{
			va_list ap;
			char buf[256];
			int n;

			if (trace_fd < 0)
				return;

			va_start(ap, fmt);
			n = vsnprintf(buf, 256, fmt, ap);
			va_end(ap);

			write(trace_fd, buf, n);
		}

	start::

		trace_fd = open("trace_marker", O_WRONLY);

	Note: Writing into the trace_marker file can also initiate triggers
	      that are written into /sys/kernel/tracing/events/ftrace/print/trigger
	      See "Event triggers" in Documentation/trace/events.rst and an
              example in Documentation/trace/histogram.rst (Section 3.)

  trace_marker_raw:

	This is similar to trace_marker above, but is meant for binary data
	to be written to it, where a tool can be used to parse the data
	from trace_pipe_raw.

  uprobe_events:

	Add dynamic tracepoints in programs.
	See uprobetracer.rst

  uprobe_profile:

	Uprobe statistics. See uprobetrace.txt

  instances:

	This is a way to make multiple trace buffers where different
	events can be recorded in different buffers.
	See "Instances" section below.

  events:

	This is the trace event directory. It holds event tracepoints
	(also known as static tracepoints) that have been compiled
	into the kernel. It shows what event tracepoints exist
	and how they are grouped by system. There are "enable"
	files at various levels that can enable the tracepoints
	when a "1" is written to them.

	See events.rst for more information.

  set_event:

	By echoing in the event into this file, will enable that event.

	See events.rst for more information.

  available_events:

	A list of events that can be enabled in tracing.

	See events.rst for more information.

  timestamp_mode:

	Certain tracers may change the timestamp mode used when
	logging trace events into the event buffer.  Events with
	different modes can coexist within a buffer but the mode in
	effect when an event is logged determines which timestamp mode
	is used for that event.  The default timestamp mode is
	'delta'.

	Usual timestamp modes for tracing:

	  # cat timestamp_mode
	  [delta] absolute

	  The timestamp mode with the square brackets around it is the
	  one in effect.

	  delta: Default timestamp mode - timestamp is a delta against
	         a per-buffer timestamp.

	  absolute: The timestamp is a full timestamp, not a delta
                 against some other value.  As such it takes up more
                 space and is less efficient.

  hwlat_detector:

	Directory for the Hardware Latency Detector.
	See "Hardware Latency Detector" section below.

  per_cpu:

	This is a directory that contains the trace per_cpu information.

  per_cpu/cpu0/buffer_size_kb:

	The ftrace buffer is defined per_cpu. That is, there's a separate
	buffer for each CPU to allow writes to be done atomically,
	and free from cache bouncing. These buffers may have different
	size buffers. This file is similar to the buffer_size_kb
	file, but it only displays or sets the buffer size for the
	specific CPU. (here cpu0).

  per_cpu/cpu0/trace:

	This is similar to the "trace" file, but it will only display
	the data specific for the CPU. If written to, it only clears
	the specific CPU buffer.

  per_cpu/cpu0/trace_pipe

	This is similar to the "trace_pipe" file, and is a consuming
	read, but it will only display (and consume) the data specific
	for the CPU.

  per_cpu/cpu0/trace_pipe_raw

	For tools that can parse the ftrace ring buffer binary format,
	the trace_pipe_raw file can be used to extract the data
	from the ring buffer directly. With the use of the splice()
	system call, the buffer data can be quickly transferred to
	a file or to the network where a server is collecting the
	data.

	Like trace_pipe, this is a consuming reader, where multiple
	reads will always produce different data.

  per_cpu/cpu0/snapshot:

	This is similar to the main "snapshot" file, but will only
	snapshot the current CPU (if supported). It only displays
	the content of the snapshot for a given CPU, and if
	written to, only clears this CPU buffer.

  per_cpu/cpu0/snapshot_raw:

	Similar to the trace_pipe_raw, but will read the binary format
	from the snapshot buffer for the given CPU.

  per_cpu/cpu0/stats:

	This displays certain stats about the ring buffer:

	entries:
		The number of events that are still in the buffer.

	overrun:
		The number of lost events due to overwriting when
		the buffer was full.

	commit overrun:
		Should always be zero.
		This gets set if so many events happened within a nested
		event (ring buffer is re-entrant), that it fills the
		buffer and starts dropping events.

	bytes:
		Bytes actually read (not overwritten).

	oldest event ts:
		The oldest timestamp in the buffer

	now ts:
		The current timestamp

	dropped events:
		Events lost due to overwrite option being off.

	read events:
		The number of events read.

The Tracers
-----------

Here is the list of current tracers that may be configured.

  "function"

	Function call tracer to trace all kernel functions.

  "function_graph"

	Similar to the function tracer except that the
	function tracer probes the functions on their entry
	whereas the function graph tracer traces on both entry
	and exit of the functions. It then provides the ability
	to draw a graph of function calls similar to C code
	source.

  "blk"

	The block tracer. The tracer used by the blktrace user
	application.

  "hwlat"

	The Hardware Latency tracer is used to detect if the hardware
	produces any latency. See "Hardware Latency Detector" section
	below.

  "irqsoff"

	Traces the areas that disable interrupts and saves
	the trace with the longest max latency.
	See tracing_max_latency. When a new max is recorded,
	it replaces the old trace. It is best to view this
	trace with the latency-format option enabled, which
	happens automatically when the tracer is selected.

  "preemptoff"

	Similar to irqsoff but traces and records the amount of
	time for which preemption is disabled.

  "preemptirqsoff"

	Similar to irqsoff and preemptoff, but traces and
	records the largest time for which irqs and/or preemption
	is disabled.

  "wakeup"

	Traces and records the max latency that it takes for
	the highest priority task to get scheduled after
	it has been woken up.
        Traces all tasks as an average developer would expect.

  "wakeup_rt"

        Traces and records the max latency that it takes for just
        RT tasks (as the current "wakeup" does). This is useful
        for those interested in wake up timings of RT tasks.

  "wakeup_dl"

	Traces and records the max latency that it takes for
	a SCHED_DEADLINE task to be woken (as the "wakeup" and
	"wakeup_rt" does).

  "mmiotrace"

	A special tracer that is used to trace binary module.
	It will trace all the calls that a module makes to the
	hardware. Everything it writes and reads from the I/O
	as well.

  "branch"

	This tracer can be configured when tracing likely/unlikely
	calls within the kernel. It will trace when a likely and
	unlikely branch is hit and if it was correct in its prediction
	of being correct.

  "nop"

	This is the "trace nothing" tracer. To remove all
	tracers from tracing simply echo "nop" into
	current_tracer.

Error conditions
----------------

  For most ftrace commands, failure modes are obvious and communicated
  using standard return codes.

  For other more involved commands, extended error information may be
  available via the tracing/error_log file.  For the commands that
  support it, reading the tracing/error_log file after an error will
  display more detailed information about what went wrong, if
  information is available.  The tracing/error_log file is a circular
  error log displaying a small number (currently, 8) of ftrace errors
  for the last (8) failed commands.

  The extended error information and usage takes the form shown in
  this example::

    # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger
    echo: write error: Invalid argument

    # cat /sys/kernel/tracing/error_log
    [ 5348.887237] location: error: Couldn't yyy: zzz
      Command: xxx
               ^
    [ 7517.023364] location: error: Bad rrr: sss
      Command: ppp qqq
                   ^

  To clear the error log, echo the empty string into it::

    # echo > /sys/kernel/tracing/error_log

Examples of using the tracer
----------------------------

Here are typical examples of using the tracers when controlling
them only with the tracefs interface (without using any
user-land utilities).

Output format:
--------------

Here is an example of the output format of the file "trace"::

  # tracer: function
  #
  # entries-in-buffer/entries-written: 140080/250280   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
              bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
              bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
              sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
              bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
              bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
              bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
              bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
              bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
              sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
              ....

A header is printed with the tracer name that is represented by
the trace. In this case the tracer is "function". Then it shows the
number of events in the buffer as well as the total number of entries
that were written. The difference is the number of entries that were
lost due to the buffer filling up (250280 - 140080 = 110200 events
lost).

The header explains the content of the events. Task name "bash", the task
PID "1977", the CPU that it was running on "000", the latency format
(explained below), the timestamp in <secs>.<usecs> format, the
function name that was traced "sys_close" and the parent function that
called this function "system_call_fastpath". The timestamp is the time
at which the function was entered.

Latency trace format
--------------------

When the latency-format option is enabled or when one of the latency
tracers is set, the trace file gives somewhat more information to see
why a latency happened. Here is a typical trace::

  # tracer: irqsoff
  #
  # irqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: __lock_task_sighand
  #  => ended at:   _raw_spin_unlock_irqrestore
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
        ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
        ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
        ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
        ps-6143    2d..1  306us : <stack trace>
   => trace_hardirqs_on_caller
   => trace_hardirqs_on
   => _raw_spin_unlock_irqrestore
   => do_task_stat
   => proc_tgid_stat
   => proc_single_show
   => seq_read
   => vfs_read
   => sys_read
   => system_call_fastpath


This shows that the current tracer is "irqsoff" tracing the time
for which interrupts were disabled. It gives the trace version (which
never changes) and the version of the kernel upon which this was executed on
(3.8). Then it displays the max latency in microseconds (259 us). The number
of trace entries displayed and the total number (both are four: #4/4).
VP, KP, SP, and HP are always zero and are reserved for later use.
#P is the number of online CPUs (#P:4).

The task is the process that was running when the latency
occurred. (ps pid: 6143).

The start and stop (the functions in which the interrupts were
disabled and enabled respectively) that caused the latencies:

  - __lock_task_sighand is where the interrupts were disabled.
  - _raw_spin_unlock_irqrestore is where they were enabled again.

The next lines after the header are the trace itself. The header
explains which is which.

  cmd: The name of the process in the trace.

  pid: The PID of that process.

  CPU#: The CPU which the process was running on.

  irqs-off: 'd' interrupts are disabled. '.' otherwise.
	.. caution:: If the architecture does not support a way to
		read the irq flags variable, an 'X' will always
		be printed here.

  need-resched:
	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
	- 'n' only TIF_NEED_RESCHED is set,
	- 'p' only PREEMPT_NEED_RESCHED is set,
	- '.' otherwise.

  hardirq/softirq:
	- 'Z' - NMI occurred inside a hardirq
	- 'z' - NMI is running
	- 'H' - hard irq occurred inside a softirq.
	- 'h' - hard irq is running
	- 's' - soft irq is running
	- '.' - normal context.

  preempt-depth: The level of preempt_disabled

The above is mostly meaningful for kernel developers.

  time:
	When the latency-format option is enabled, the trace file
	output includes a timestamp relative to the start of the
	trace. This differs from the output when latency-format
	is disabled, which includes an absolute timestamp.

  delay:
	This is just to help catch your eye a bit better. And
	needs to be fixed to be only relative to the same CPU.
	The marks are determined by the difference between this
	current trace and the next trace.

	  - '$' - greater than 1 second
	  - '@' - greater than 100 millisecond
	  - '*' - greater than 10 millisecond
	  - '#' - greater than 1000 microsecond
	  - '!' - greater than 100 microsecond
	  - '+' - greater than 10 microsecond
	  - ' ' - less than or equal to 10 microsecond.

  The rest is the same as the 'trace' file.

  Note, the latency tracers will usually end with a back trace
  to easily find where the latency occurred.

trace_options
-------------

The trace_options file (or the options directory) is used to control
what gets printed in the trace output, or manipulate the tracers.
To see what is available, simply cat the file::

  cat trace_options
	print-parent
	nosym-offset
	nosym-addr
	noverbose
	noraw
	nohex
	nobin
	noblock
	nofields
	trace_printk
	annotate
	nouserstacktrace
	nosym-userobj
	noprintk-msg-only
	context-info
	nolatency-format
	record-cmd
	norecord-tgid
	overwrite
	nodisable_on_free
	irq-info
	markers
	noevent-fork
	function-trace
	nofunction-fork
	nodisplay-graph
	nostacktrace
	nobranch

To disable one of the options, echo in the option prepended with
"no"::

  echo noprint-parent > trace_options

To enable an option, leave off the "no"::

  echo sym-offset > trace_options

Here are the available options:

  print-parent
	On function traces, display the calling (parent)
	function as well as the function being traced.
	::

	  print-parent:
	   bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul

	  noprint-parent:
	   bash-4000  [01]  1477.606694: simple_strtoul


  sym-offset
	Display not only the function name, but also the
	offset in the function. For example, instead of
	seeing just "ktime_get", you will see
	"ktime_get+0xb/0x20".
	::

	  sym-offset:
	   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0

  sym-addr
	This will also display the function address as well
	as the function name.
	::

	  sym-addr:
	   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>

  verbose
	This deals with the trace file when the
        latency-format option is enabled.
	::

	    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
	    (+0.000ms): simple_strtoul (kstrtoul)

  raw
	This will display raw numbers. This option is best for
	use with user applications that can translate the raw
	numbers better than having it done in the kernel.

  hex
	Similar to raw, but the numbers will be in a hexadecimal format.

  bin
	This will print out the formats in raw binary.

  block
	When set, reading trace_pipe will not block when polled.

  fields
	Print the fields as described by their types. This is a better
	option than using hex, bin or raw, as it gives a better parsing
	of the content of the event.

  trace_printk
	Can disable trace_printk() from writing into the buffer.

  annotate
	It is sometimes confusing when the CPU buffers are full
	and one CPU buffer had a lot of events recently, thus
	a shorter time frame, were another CPU may have only had
	a few events, which lets it have older events. When
	the trace is reported, it shows the oldest events first,
	and it may look like only one CPU ran (the one with the
	oldest events). When the annotate option is set, it will
	display when a new CPU buffer started::

			  <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
			  <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
			  <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
		##### CPU 2 buffer started ####
			  <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
			  <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
			  <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock

  userstacktrace
	This option changes the trace. It records a
	stacktrace of the current user space thread after
	each trace event.

  sym-userobj
	when user stacktrace are enabled, look up which
	object the address belongs to, and print a
	relative address. This is especially useful when
	ASLR is on, otherwise you don't get a chance to
	resolve the address to object/file/line after
	the app is no longer running

	The lookup is performed when you read
	trace,trace_pipe. Example::

		  a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
		  x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]


  printk-msg-only
	When set, trace_printk()s will only show the format
	and not their parameters (if trace_bprintk() or
	trace_bputs() was used to save the trace_printk()).

  context-info
	Show only the event data. Hides the comm, PID,
	timestamp, CPU, and other useful data.

  latency-format
	This option changes the trace output. When it is enabled,
	the trace displays additional information about the
	latency, as described in "Latency trace format".

  pause-on-trace
	When set, opening the trace file for read, will pause
	writing to the ring buffer (as if tracing_on was set to zero).
	This simulates the original behavior of the trace file.
	When the file is closed, tracing will be enabled again.

  hash-ptr
        When set, "%p" in the event printk format displays the
        hashed pointer value instead of real address.
        This will be useful if you want to find out which hashed
        value is corresponding to the real value in trace log.

  record-cmd
	When any event or tracer is enabled, a hook is enabled
	in the sched_switch trace point to fill comm cache
	with mapped pids and comms. But this may cause some
	overhead, and if you only care about pids, and not the
	name of the task, disabling this option can lower the
	impact of tracing. See "saved_cmdlines".

  record-tgid
	When any event or tracer is enabled, a hook is enabled
	in the sched_switch trace point to fill the cache of
	mapped Thread Group IDs (TGID) mapping to pids. See
	"saved_tgids".

  overwrite
	This controls what happens when the trace buffer is
	full. If "1" (default), the oldest events are
	discarded and overwritten. If "0", then the newest
	events are discarded.
	(see per_cpu/cpu0/stats for overrun and dropped)

  disable_on_free
	When the free_buffer is closed, tracing will
	stop (tracing_on set to 0).

  irq-info
	Shows the interrupt, preempt count, need resched data.
	When disabled, the trace looks like::

		# tracer: function
		#
		# entries-in-buffer/entries-written: 144405/9452052   #P:4
		#
		#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
		#              | |       |          |         |
			  <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
			  <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
			  <idle>-0     [002]  23636.756055: enqueue_task <-activate_task


  markers
	When set, the trace_marker is writable (only by root).
	When disabled, the trace_marker will error with EINVAL
	on write.

  event-fork
	When set, tasks with PIDs listed in set_event_pid will have
	the PIDs of their children added to set_event_pid when those
	tasks fork. Also, when tasks with PIDs in set_event_pid exit,
	their PIDs will be removed from the file.

        This affects PIDs listed in set_event_notrace_pid as well.

  function-trace
	The latency tracers will enable function tracing
	if this option is enabled (default it is). When
	it is disabled, the latency tracers do not trace
	functions. This keeps the overhead of the tracer down
	when performing latency tests.

  function-fork
	When set, tasks with PIDs listed in set_ftrace_pid will
	have the PIDs of their children added to set_ftrace_pid
	when those tasks fork. Also, when tasks with PIDs in
	set_ftrace_pid exit, their PIDs will be removed from the
	file.

        This affects PIDs in set_ftrace_notrace_pid as well.

  display-graph
	When set, the latency tracers (irqsoff, wakeup, etc) will
	use function graph tracing instead of function tracing.

  stacktrace
	When set, a stack trace is recorded after any trace event
	is recorded.

  branch
	Enable branch tracing with the tracer. This enables branch
	tracer along with the currently set tracer. Enabling this
	with the "nop" tracer is the same as just enabling the
	"branch" tracer.

.. tip:: Some tracers have their own options. They only appear in this
       file when the tracer is active. They always appear in the
       options directory.


Here are the per tracer options:

Options for function tracer:

  func_stack_trace
	When set, a stack trace is recorded after every
	function that is recorded. NOTE! Limit the functions
	that are recorded before enabling this, with
	"set_ftrace_filter" otherwise the system performance
	will be critically degraded. Remember to disable
	this option before clearing the function filter.

Options for function_graph tracer:

 Since the function_graph tracer has a slightly different output
 it has its own options to control what is displayed.

  funcgraph-overrun
	When set, the "overrun" of the graph stack is
	displayed after each function traced. The
	overrun, is when the stack depth of the calls
	is greater than what is reserved for each task.
	Each task has a fixed array of functions to
	trace in the call graph. If the depth of the
	calls exceeds that, the function is not traced.
	The overrun is the number of functions missed
	due to exceeding this array.

  funcgraph-cpu
	When set, the CPU number of the CPU where the trace
	occurred is displayed.

  funcgraph-overhead
	When set, if the function takes longer than
	A certain amount, then a delay marker is
	displayed. See "delay" above, under the
	header description.

  funcgraph-proc
	Unlike other tracers, the process' command line
	is not displayed by default, but instead only
	when a task is traced in and out during a context
	switch. Enabling this options has the command
	of each process displayed at every line.

  funcgraph-duration
	At the end of each function (the return)
	the duration of the amount of time in the
	function is displayed in microseconds.

  funcgraph-abstime
	When set, the timestamp is displayed at each line.

  funcgraph-irqs
	When disabled, functions that happen inside an
	interrupt will not be traced.

  funcgraph-tail
	When set, the return event will include the function
	that it represents. By default this is off, and
	only a closing curly bracket "}" is displayed for
	the return of a function.

  funcgraph-retval
	When set, the return value of each traced function
	will be printed after an equal sign "=". By default
	this is off.

  funcgraph-retval-hex
	When set, the return value will always be printed
	in hexadecimal format. If the option is not set and
	the return value is an error code, it will be printed
	in signed decimal format; otherwise it will also be
	printed in hexadecimal format. By default, this option
	is off.

  sleep-time
	When running function graph tracer, to include
	the time a task schedules out in its function.
	When enabled, it will account time the task has been
	scheduled out as part of the function call.

  graph-time
	When running function profiler with function graph tracer,
	to include the time to call nested functions. When this is
	not set, the time reported for the function will only
	include the time the function itself executed for, not the
	time for functions that it called.

Options for blk tracer:

  blk_classic
	Shows a more minimalistic output.


irqsoff
-------

When interrupts are disabled, the CPU can not react to any other
external event (besides NMIs and SMIs). This prevents the timer
interrupt from triggering or the mouse interrupt from letting
the kernel know of a new mouse event. The result is a latency
with the reaction time.

The irqsoff tracer tracks the time for which interrupts are
disabled. When a new maximum latency is hit, the tracer saves
the trace leading up to that latency point so that every time a
new maximum is reached, the old saved trace is discarded and the
new trace is saved.

To reset the maximum, echo 0 into tracing_max_latency. Here is
an example::

  # echo 0 > options/function-trace
  # echo irqsoff > current_tracer
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat trace
  # tracer: irqsoff
  #
  # irqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: run_timer_softirq
  #  => ended at:   run_timer_softirq
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
    <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
    <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
    <idle>-0       0dNs3   25us : <stack trace>
   => _raw_spin_unlock_irq
   => run_timer_softirq
   => __do_softirq
   => call_softirq
   => do_softirq
   => irq_exit
   => smp_apic_timer_interrupt
   => apic_timer_interrupt
   => rcu_idle_exit
   => cpu_idle
   => rest_init
   => start_kernel
   => x86_64_start_reservations
   => x86_64_start_kernel

Here we see that we had a latency of 16 microseconds (which is
very good). The _raw_spin_lock_irq in run_timer_softirq disabled
interrupts. The difference between the 16 and the displayed
timestamp 25us occurred because the clock was incremented
between the time of recording the max latency and the time of
recording the function that had that latency.

Note the above example had function-trace not set. If we set
function-trace, we get a much larger output::

 with echo 1 > options/function-trace

  # tracer: irqsoff
  #
  # irqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: ata_scsi_queuecmd
  #  => ended at:   ata_scsi_queuecmd
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
      bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
      bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
      bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
      bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
      bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
      bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
      bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
      bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
  [...]
      bash-2042    3d..1   67us : delay_tsc <-__delay
      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
      bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
      bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
      bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
      bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
      bash-2042    3d..1  120us : <stack trace>
   => _raw_spin_unlock_irqrestore
   => ata_scsi_queuecmd
   => scsi_dispatch_cmd
   => scsi_request_fn
   => __blk_run_queue_uncond
   => __blk_run_queue
   => blk_queue_bio
   => submit_bio_noacct
   => submit_bio
   => submit_bh
   => __ext3_get_inode_loc
   => ext3_iget
   => ext3_lookup
   => lookup_real
   => __lookup_hash
   => walk_component
   => lookup_last
   => path_lookupat
   => filename_lookup
   => user_path_at_empty
   => user_path_at
   => vfs_fstatat
   => vfs_stat
   => sys_newstat
   => system_call_fastpath


Here we traced a 71 microsecond latency. But we also see all the
functions that were called during that time. Note that by
enabling function tracing, we incur an added overhead. This
overhead may extend the latency times. But nevertheless, this
trace has provided some very helpful debugging information.

If we prefer function graph output instead of function, we can set
display-graph option::

 with echo 1 > options/display-graph

  # tracer: irqsoff
  #
  # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
  # --------------------------------------------------------------------
  # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: free_debug_processing
  #  => ended at:   return_to_handler
  #
  #
  #                                       _-----=> irqs-off
  #                                      / _----=> need-resched
  #                                     | / _---=> hardirq/softirq
  #                                     || / _--=> preempt-depth
  #                                     ||| /
  #   REL TIME      CPU  TASK/PID       ||||     DURATION                  FUNCTION CALLS
  #      |          |     |    |        ||||      |   |                     |   |   |   |
          0 us |   0)   bash-1507    |  d... |   0.000 us    |  _raw_spin_lock_irqsave();
          0 us |   0)   bash-1507    |  d..1 |   0.378 us    |    do_raw_spin_trylock();
          1 us |   0)   bash-1507    |  d..2 |               |    set_track() {
          2 us |   0)   bash-1507    |  d..2 |               |      save_stack_trace() {
          2 us |   0)   bash-1507    |  d..2 |               |        __save_stack_trace() {
          3 us |   0)   bash-1507    |  d..2 |               |          __unwind_start() {
          3 us |   0)   bash-1507    |  d..2 |               |            get_stack_info() {
          3 us |   0)   bash-1507    |  d..2 |   0.351 us    |              in_task_stack();
          4 us |   0)   bash-1507    |  d..2 |   1.107 us    |            }
  [...]
       3750 us |   0)   bash-1507    |  d..1 |   0.516 us    |      do_raw_spin_unlock();
       3750 us |   0)   bash-1507    |  d..1 |   0.000 us    |  _raw_spin_unlock_irqrestore();
       3764 us |   0)   bash-1507    |  d..1 |   0.000 us    |  tracer_hardirqs_on();
      bash-1507    0d..1 3792us : <stack trace>
   => free_debug_processing
   => __slab_free
   => kmem_cache_free
   => vm_area_free
   => remove_vma
   => exit_mmap
   => mmput
   => begin_new_exec
   => load_elf_binary
   => search_binary_handler
   => __do_execve_file.isra.32
   => __x64_sys_execve
   => do_syscall_64
   => entry_SYSCALL_64_after_hwframe

preemptoff
----------

When preemption is disabled, we may be able to receive
interrupts but the task cannot be preempted and a higher
priority task must wait for preemption to be enabled again
before it can preempt a lower priority task.

The preemptoff tracer traces the places that disable preemption.
Like the irqsoff tracer, it records the maximum latency for
which preemption was disabled. The control of preemptoff tracer
is much like the irqsoff tracer.
::

  # echo 0 > options/function-trace
  # echo preemptoff > current_tracer
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat trace
  # tracer: preemptoff
  #
  # preemptoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: do_IRQ
  #  => ended at:   do_IRQ
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
      sshd-1991    1d..1   46us : irq_exit <-do_IRQ
      sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
      sshd-1991    1d..1   52us : <stack trace>
   => sub_preempt_count
   => irq_exit
   => do_IRQ
   => ret_from_intr


This has some more changes. Preemption was disabled when an
interrupt came in (notice the 'h'), and was enabled on exit.
But we also see that interrupts have been disabled when entering
the preempt off section and leaving it (the 'd'). We do not know if
interrupts were enabled in the mean time or shortly after this
was over.
::

  # tracer: preemptoff
  #
  # preemptoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: wake_up_new_task
  #  => ended at:   task_rq_unlock
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
      bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
      bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
      bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
  [...]
      bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
      bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
      bash-1994    1d..1   13us : add_preempt_count <-irq_enter
      bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
      bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
      bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
      bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
      bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
  [...]
      bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
      bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
      bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
      bash-1994    1d..2   36us : do_softirq <-irq_exit
      bash-1994    1d..2   36us : __do_softirq <-call_softirq
      bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
      bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
      bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
      bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
      bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
  [...]
      bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
      bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
      bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
      bash-1994    1dN.2   82us : idle_cpu <-irq_exit
      bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
      bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
      bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
      bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
      bash-1994    1.N.1  104us : <stack trace>
   => sub_preempt_count
   => _raw_spin_unlock_irqrestore
   => task_rq_unlock
   => wake_up_new_task
   => do_fork
   => sys_clone
   => stub_clone


The above is an example of the preemptoff trace with
function-trace set. Here we see that interrupts were not disabled
the entire time. The irq_enter code lets us know that we entered
an interrupt 'h'. Before that, the functions being traced still
show that it is not in an interrupt, but we can see from the
functions themselves that this is not the case.

preemptirqsoff
--------------

Knowing the locations that have interrupts disabled or
preemption disabled for the longest times is helpful. But
sometimes we would like to know when either preemption and/or
interrupts are disabled.

Consider the following code::

    local_irq_disable();
    call_function_with_irqs_off();
    preempt_disable();
    call_function_with_irqs_and_preemption_off();
    local_irq_enable();
    call_function_with_preemption_off();
    preempt_enable();

The irqsoff tracer will record the total length of
call_function_with_irqs_off() and
call_function_with_irqs_and_preemption_off().

The preemptoff tracer will record the total length of
call_function_with_irqs_and_preemption_off() and
call_function_with_preemption_off().

But neither will trace the time that interrupts and/or
preemption is disabled. This total time is the time that we can
not schedule. To record this time, use the preemptirqsoff
tracer.

Again, using this trace is much like the irqsoff and preemptoff
tracers.
::

  # echo 0 > options/function-trace
  # echo preemptirqsoff > current_tracer
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # ls -ltr
  [...]
  # echo 0 > tracing_on
  # cat trace
  # tracer: preemptirqsoff
  #
  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: ata_scsi_queuecmd
  #  => ended at:   ata_scsi_queuecmd
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
        ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
        ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
        ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
        ls-2230    3...1  111us : <stack trace>
   => sub_preempt_count
   => _raw_spin_unlock_irqrestore
   => ata_scsi_queuecmd
   => scsi_dispatch_cmd
   => scsi_request_fn
   => __blk_run_queue_uncond
   => __blk_run_queue
   => blk_queue_bio
   => submit_bio_noacct
   => submit_bio
   => submit_bh
   => ext3_bread
   => ext3_dir_bread
   => htree_dirblock_to_tree
   => ext3_htree_fill_tree
   => ext3_readdir
   => vfs_readdir
   => sys_getdents
   => system_call_fastpath


The trace_hardirqs_off_thunk is called from assembly on x86 when
interrupts are disabled in the assembly code. Without the
function tracing, we do not know if interrupts were enabled
within the preemption points. We do see that it started with
preemption enabled.

Here is a trace with function-trace set::

  # tracer: preemptirqsoff
  #
  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
  #    -----------------
  #  => started at: schedule
  #  => ended at:   mutex_unlock
  #
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
  kworker/-59      3...1    0us : __schedule <-schedule
  kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
  kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
  kworker/-59      3d..2    1us : deactivate_task <-__schedule
  kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
  kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
  kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
  kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
  kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
  kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
  kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
  kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
  kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
  kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
  kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
  kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
  kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
  kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
  kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
  kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
  kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
  kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
  kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
  kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
  kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
        ls-2269    3d..2    7us : finish_task_switch <-__schedule
        ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
        ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
        ls-2269    3d..2    8us : irq_enter <-do_IRQ
        ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
        ls-2269    3d..2    9us : add_preempt_count <-irq_enter
        ls-2269    3d.h2    9us : exit_idle <-do_IRQ
  [...]
        ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
        ls-2269    3d.h2   20us : irq_exit <-do_IRQ
        ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
        ls-2269    3d..3   21us : do_softirq <-irq_exit
        ls-2269    3d..3   21us : __do_softirq <-call_softirq
        ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
        ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
        ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
        ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
        ls-2269    3d.s5   31us : irq_enter <-do_IRQ
        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
  [...]
        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
        ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
        ls-2269    3d.H5   32us : exit_idle <-do_IRQ
        ls-2269    3d.H5   32us : handle_irq <-do_IRQ
        ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
        ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
  [...]
        ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
        ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
        ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
        ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
        ls-2269    3d..3  159us : idle_cpu <-irq_exit
        ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
        ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
        ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
        ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
        ls-2269    3d...  186us : <stack trace>
   => __mutex_unlock_slowpath
   => mutex_unlock
   => process_output
   => n_tty_write
   => tty_write
   => vfs_write
   => sys_write
   => system_call_fastpath

This is an interesting trace. It started with kworker running and
scheduling out and ls taking over. But as soon as ls released the
rq lock and enabled interrupts (but not preemption) an interrupt
triggered. When the interrupt finished, it started running softirqs.
But while the softirq was running, another interrupt triggered.
When an interrupt is running inside a softirq, the annotation is 'H'.


wakeup
------

One common case that people are interested in tracing is the
time it takes for a task that is woken to actually wake up.
Now for non Real-Time tasks, this can be arbitrary. But tracing
it none the less can be interesting. 

Without function tracing::

  # echo 0 > options/function-trace
  # echo wakeup > current_tracer
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # chrt -f 5 sleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: wakeup
  #
  # wakeup latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
  #    -----------------
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
    <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d..3   15us : __schedule <-schedule
    <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H

The tracer only traces the highest priority task in the system
to avoid tracing the normal circumstances. Here we see that
the kworker with a nice priority of -20 (not very nice), took
just 15 microseconds from the time it woke up, to the time it
ran.

Non Real-Time tasks are not that interesting. A more interesting
trace is to concentrate only on Real-Time tasks.

wakeup_rt
---------

In a Real-Time environment it is very important to know the
wakeup time it takes for the highest priority task that is woken
up to the time that it executes. This is also known as "schedule
latency". I stress the point that this is about RT tasks. It is
also important to know the scheduling latency of non-RT tasks,
but the average schedule latency is better for non-RT tasks.
Tools like LatencyTop are more appropriate for such
measurements.

Real-Time environments are interested in the worst case latency.
That is the longest latency it takes for something to happen,
and not the average. We can have a very fast scheduler that may
only have a large latency once in a while, but that would not
work well with Real-Time tasks.  The wakeup_rt tracer was designed
to record the worst case wakeups of RT tasks. Non-RT tasks are
not recorded because the tracer only records one worst case and
tracing non-RT tasks that are unpredictable will overwrite the
worst case latency of RT tasks (just run the normal wakeup
tracer for a while to see that effect).

Since this tracer only deals with RT tasks, we will run this
slightly differently than we did with the previous tracers.
Instead of performing an 'ls', we will run 'sleep 1' under
'chrt' which changes the priority of the task.
::

  # echo 0 > options/function-trace
  # echo wakeup_rt > current_tracer
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # chrt -f 5 sleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: wakeup
  #
  # tracer: wakeup_rt
  #
  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
  #    -----------------
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
    <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d..3    5us : __schedule <-schedule
    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep


Running this on an idle system, we see that it only took 5 microseconds
to perform the task switch.  Note, since the trace point in the schedule
is before the actual "switch", we stop the tracing when the recorded task
is about to schedule in. This may change if we add a new marker at the
end of the scheduler.

Notice that the recorded task is 'sleep' with the PID of 2389
and it has an rt_prio of 5. This priority is user-space priority
and not the internal kernel priority. The policy is 1 for
SCHED_FIFO and 2 for SCHED_RR.

Note, that the trace data shows the internal priority (99 - rtprio).
::

  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep

The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
and in the running state 'R'. The sleep task was scheduled in with
2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
and it too is in the running state.

Doing the same with chrt -r 5 and function-trace set.
::

  echo 1 > options/function-trace

  # tracer: wakeup_rt
  #
  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
  #    -----------------
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
    <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
    <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
    <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
    <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
    <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
    <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
    <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
    <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
    <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
    <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
    <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
    <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
    <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
    <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
    <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
    <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
    <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
    <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
    <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
    <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
    <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
    <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
    <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
    <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
    <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
    <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
    <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
    <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
    <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
    <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
    <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
    <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
    <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
    <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit
    <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
    <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
    <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
    <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
    <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
    <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
    <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
    <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
    <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
    <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
    <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
    <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
    <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
    <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
    <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
    <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
    <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
    <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
    <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
    <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
    <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
    <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
    <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
    <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
    <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
    <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
    <idle>-0       3.N..   25us : schedule <-cpu_idle
    <idle>-0       3.N..   25us : __schedule <-preempt_schedule
    <idle>-0       3.N..   26us : add_preempt_count <-__schedule
    <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
    <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
    <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
    <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
    <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
    <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
    <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
    <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
    <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
    <idle>-0       3d..3   29us : __schedule <-preempt_schedule
    <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep

This isn't that big of a trace, even with function tracing enabled,
so I included the entire trace.

The interrupt went off while when the system was idle. Somewhere
before task_woken_rt() was called, the NEED_RESCHED flag was set,
this is indicated by the first occurrence of the 'N' flag.

Latency tracing and events
--------------------------
As function tracing can induce a much larger latency, but without
seeing what happens within the latency it is hard to know what
caused it. There is a middle ground, and that is with enabling
events.
::

  # echo 0 > options/function-trace
  # echo wakeup_rt > current_tracer
  # echo 1 > events/enable
  # echo 1 > tracing_on
  # echo 0 > tracing_max_latency
  # chrt -f 5 sleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: wakeup_rt
  #
  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
  # --------------------------------------------------------------------
  # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
  #    -----------------
  #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
  #    -----------------
  #
  #                  _------=> CPU#            
  #                 / _-----=> irqs-off        
  #                | / _----=> need-resched    
  #                || / _---=> hardirq/softirq 
  #                ||| / _--=> preempt-depth   
  #                |||| /     delay             
  #  cmd     pid   ||||| time  |   caller      
  #     \   /      |||||  \    |   /           
    <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
    <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
    <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
    <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
    <idle>-0       2.N.2    2us : power_end: cpu_id=2
    <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
    <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
    <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
    <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
    <idle>-0       2.N.2    5us : rcu_utilization: End context switch
    <idle>-0       2d..3    6us : __schedule <-schedule
    <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep


Hardware Latency Detector
-------------------------

The hardware latency detector is executed by enabling the "hwlat" tracer.

NOTE, this tracer will affect the performance of the system as it will
periodically make a CPU constantly busy with interrupts disabled.
::

  # echo hwlat > current_tracer
  # sleep 100
  # cat trace
  # tracer: hwlat
  #
  # entries-in-buffer/entries-written: 13/13   #P:8
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
             <...>-1729  [001] d...   678.473449: #1     inner/outer(us):   11/12    ts:1581527483.343962693 count:6
             <...>-1729  [004] d...   689.556542: #2     inner/outer(us):   16/9     ts:1581527494.889008092 count:1
             <...>-1729  [005] d...   714.756290: #3     inner/outer(us):   16/16    ts:1581527519.678961629 count:5
             <...>-1729  [001] d...   718.788247: #4     inner/outer(us):    9/17    ts:1581527523.889012713 count:1
             <...>-1729  [002] d...   719.796341: #5     inner/outer(us):   13/9     ts:1581527524.912872606 count:1
             <...>-1729  [006] d...   844.787091: #6     inner/outer(us):    9/12    ts:1581527649.889048502 count:2
             <...>-1729  [003] d...   849.827033: #7     inner/outer(us):   18/9     ts:1581527654.889013793 count:1
             <...>-1729  [007] d...   853.859002: #8     inner/outer(us):    9/12    ts:1581527658.889065736 count:1
             <...>-1729  [001] d...   855.874978: #9     inner/outer(us):    9/11    ts:1581527660.861991877 count:1
             <...>-1729  [001] d...   863.938932: #10    inner/outer(us):    9/11    ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
             <...>-1729  [007] d...   878.050780: #11    inner/outer(us):    9/12    ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
             <...>-1729  [007] d...   886.114702: #12    inner/outer(us):    9/12    ts:1581527691.385001600 count:1


The above output is somewhat the same in the header. All events will have
interrupts disabled 'd'. Under the FUNCTION title there is:

 #1
	This is the count of events recorded that were greater than the
	tracing_threshold (See below).

 inner/outer(us):   11/11

      This shows two numbers as "inner latency" and "outer latency". The test
      runs in a loop checking a timestamp twice. The latency detected within
      the two timestamps is the "inner latency" and the latency detected
      after the previous timestamp and the next timestamp in the loop is
      the "outer latency".

 ts:1581527483.343962693

      The absolute timestamp that the first latency was recorded in the window.

 count:6

      The number of times a latency was detected during the window.

 nmi-total:7 nmi-count:1

      On architectures that support it, if an NMI comes in during the
      test, the time spent in NMI is reported in "nmi-total" (in
      microseconds).

      All architectures that have NMIs will show the "nmi-count" if an
      NMI comes in during the test.

hwlat files:

  tracing_threshold
	This gets automatically set to "10" to represent 10
	microseconds. This is the threshold of latency that
	needs to be detected before the trace will be recorded.

	Note, when hwlat tracer is finished (another tracer is
	written into "current_tracer"), the original value for
	tracing_threshold is placed back into this file.

  hwlat_detector/width
	The length of time the test runs with interrupts disabled.

  hwlat_detector/window
	The length of time of the window which the test
	runs. That is, the test will run for "width"
	microseconds per "window" microseconds

  tracing_cpumask
	When the test is started. A kernel thread is created that
	runs the test. This thread will alternate between CPUs
	listed in the tracing_cpumask between each period
	(one "window"). To limit the test to specific CPUs
	set the mask in this file to only the CPUs that the test
	should run on.

function
--------

This tracer is the function tracer. Enabling the function tracer
can be done from the debug file system. Make sure the
ftrace_enabled is set; otherwise this tracer is a nop.
See the "ftrace_enabled" section below.
::

  # sysctl kernel.ftrace_enabled=1
  # echo function > current_tracer
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: function
  #
  # entries-in-buffer/entries-written: 24799/24799   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
              bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
              bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
              bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
              bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
              bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
              bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
              bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
  [...]


Note: function tracer uses ring buffers to store the above
entries. The newest data may overwrite the oldest data.
Sometimes using echo to stop the trace is not sufficient because
the tracing could have overwritten the data that you wanted to
record. For this reason, it is sometimes better to disable
tracing directly from a program. This allows you to stop the
tracing at the point that you hit the part that you are
interested in. To disable the tracing directly from a C program,
something like following code snippet can be used::

	int trace_fd;
	[...]
	int main(int argc, char *argv[]) {
		[...]
		trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
		[...]
		if (condition_hit()) {
			write(trace_fd, "0", 1);
		}
		[...]
	}


Single thread tracing
---------------------

By writing into set_ftrace_pid you can trace a
single thread. For example::

  # cat set_ftrace_pid
  no pid
  # echo 3111 > set_ftrace_pid
  # cat set_ftrace_pid
  3111
  # echo function > current_tracer
  # cat trace | head
  # tracer: function
  #
  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
  #              | |       |          |         |
      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
  # echo > set_ftrace_pid
  # cat trace |head
  # tracer: function
  #
  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
  #              | |       |          |         |
  ##### CPU 3 buffer started ####
      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit

If you want to trace a function when executing, you could use
something like this simple program.
::

	#include <stdio.h>
	#include <stdlib.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>
	#include <unistd.h>
	#include <string.h>

	#define _STR(x) #x
	#define STR(x) _STR(x)
	#define MAX_PATH 256

	const char *find_tracefs(void)
	{
	       static char tracefs[MAX_PATH+1];
	       static int tracefs_found;
	       char type[100];
	       FILE *fp;

	       if (tracefs_found)
		       return tracefs;

	       if ((fp = fopen("/proc/mounts","r")) == NULL) {
		       perror("/proc/mounts");
		       return NULL;
	       }

	       while (fscanf(fp, "%*s %"
		             STR(MAX_PATH)
		             "s %99s %*s %*d %*d\n",
		             tracefs, type) == 2) {
		       if (strcmp(type, "tracefs") == 0)
		               break;
	       }
	       fclose(fp);

	       if (strcmp(type, "tracefs") != 0) {
		       fprintf(stderr, "tracefs not mounted");
		       return NULL;
	       }

	       strcat(tracefs, "/tracing/");
	       tracefs_found = 1;

	       return tracefs;
	}

	const char *tracing_file(const char *file_name)
	{
	       static char trace_file[MAX_PATH+1];
	       snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
	       return trace_file;
	}

	int main (int argc, char **argv)
	{
		if (argc < 1)
		        exit(-1);

		if (fork() > 0) {
		        int fd, ffd;
		        char line[64];
		        int s;

		        ffd = open(tracing_file("current_tracer"), O_WRONLY);
		        if (ffd < 0)
		                exit(-1);
		        write(ffd, "nop", 3);

		        fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
		        s = sprintf(line, "%d\n", getpid());
		        write(fd, line, s);

		        write(ffd, "function", 8);

		        close(fd);
		        close(ffd);

		        execvp(argv[1], argv+1);
		}

		return 0;
	}

Or this simple script!
::

  #!/bin/bash

  tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
  echo 0 > $tracefs/tracing_on
  echo $$ > $tracefs/set_ftrace_pid
  echo function > $tracefs/current_tracer
  echo 1 > $tracefs/tracing_on
  exec "$@"


function graph tracer
---------------------------

This tracer is similar to the function tracer except that it
probes a function on its entry and its exit. This is done by
using a dynamically allocated stack of return addresses in each
task_struct. On function entry the tracer overwrites the return
address of each function traced to set a custom probe. Thus the
original return address is stored on the stack of return address
in the task_struct.

Probing on both ends of a function leads to special features
such as:

- measure of a function's time execution
- having a reliable call stack to draw function calls graph

This tracer is useful in several situations:

- you want to find the reason of a strange kernel behavior and
  need to see what happens in detail on any areas (or specific
  ones).

- you are experiencing weird latencies but it's difficult to
  find its origin.

- you want to find quickly which path is taken by a specific
  function

- you just want to peek inside a working kernel and want to see
  what happens there.

::

  # tracer: function_graph
  #
  # CPU  DURATION                  FUNCTION CALLS
  # |     |   |                     |   |   |   |

   0)               |  sys_open() {
   0)               |    do_sys_open() {
   0)               |      getname() {
   0)               |        kmem_cache_alloc() {
   0)   1.382 us    |          __might_sleep();
   0)   2.478 us    |        }
   0)               |        strncpy_from_user() {
   0)               |          might_fault() {
   0)   1.389 us    |            __might_sleep();
   0)   2.553 us    |          }
   0)   3.807 us    |        }
   0)   7.876 us    |      }
   0)               |      alloc_fd() {
   0)   0.668 us    |        _spin_lock();
   0)   0.570 us    |        expand_files();
   0)   0.586 us    |        _spin_unlock();


There are several columns that can be dynamically
enabled/disabled. You can use every combination of options you
want, depending on your needs.

- The cpu number on which the function executed is default
  enabled.  It is sometimes better to only trace one cpu (see
  tracing_cpumask file) or you might sometimes see unordered
  function calls while cpu tracing switch.

	- hide: echo nofuncgraph-cpu > trace_options
	- show: echo funcgraph-cpu > trace_options

- The duration (function's time of execution) is displayed on
  the closing bracket line of a function or on the same line
  than the current function in case of a leaf one. It is default
  enabled.

	- hide: echo nofuncgraph-duration > trace_options
	- show: echo funcgraph-duration > trace_options

- The overhead field precedes the duration field in case of
  reached duration thresholds.

	- hide: echo nofuncgraph-overhead > trace_options
	- show: echo funcgraph-overhead > trace_options
	- depends on: funcgraph-duration

  ie::

    3) # 1837.709 us |          } /* __switch_to */
    3)               |          finish_task_switch() {
    3)   0.313 us    |            _raw_spin_unlock_irq();
    3)   3.177 us    |          }
    3) # 1889.063 us |        } /* __schedule */
    3) ! 140.417 us  |      } /* __schedule */
    3) # 2034.948 us |    } /* schedule */
    3) * 33998.59 us |  } /* schedule_preempt_disabled */

    [...]

    1)   0.260 us    |              msecs_to_jiffies();
    1)   0.313 us    |              __rcu_read_unlock();
    1) + 61.770 us   |            }
    1) + 64.479 us   |          }
    1)   0.313 us    |          rcu_bh_qs();
    1)   0.313 us    |          __local_bh_enable();
    1) ! 217.240 us  |        }
    1)   0.365 us    |        idle_cpu();
    1)               |        rcu_irq_exit() {
    1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
    1)   3.125 us    |        }
    1) ! 227.812 us  |      }
    1) ! 457.395 us  |    }
    1) @ 119760.2 us |  }

    [...]

    2)               |    handle_IPI() {
    1)   6.979 us    |                  }
    2)   0.417 us    |      scheduler_ipi();
    1)   9.791 us    |                }
    1) + 12.917 us   |              }
    2)   3.490 us    |    }
    1) + 15.729 us   |            }
    1) + 18.542 us   |          }
    2) $ 3594274 us  |  }

Flags::

  + means that the function exceeded 10 usecs.
  ! means that the function exceeded 100 usecs.
  # means that the function exceeded 1000 usecs.
  * means that the function exceeded 10 msecs.
  @ means that the function exceeded 100 msecs.
  $ means that the function exceeded 1 sec.


- The task/pid field displays the thread cmdline and pid which
  executed the function. It is default disabled.

	- hide: echo nofuncgraph-proc > trace_options
	- show: echo funcgraph-proc > trace_options

  ie::

    # tracer: function_graph
    #
    # CPU  TASK/PID        DURATION                  FUNCTION CALLS
    # |    |    |           |   |                     |   |   |   |
    0)    sh-4802     |               |                  d_free() {
    0)    sh-4802     |               |                    call_rcu() {
    0)    sh-4802     |               |                      __call_rcu() {
    0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
    0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
    0)    sh-4802     |   2.899 us    |                      }
    0)    sh-4802     |   4.040 us    |                    }
    0)    sh-4802     |   5.151 us    |                  }
    0)    sh-4802     | + 49.370 us   |                }


- The absolute time field is an absolute timestamp given by the
  system clock since it started. A snapshot of this time is
  given on each entry/exit of functions

	- hide: echo nofuncgraph-abstime > trace_options
	- show: echo funcgraph-abstime > trace_options

  ie::

    #
    #      TIME       CPU  DURATION                  FUNCTION CALLS
    #       |         |     |   |                     |   |   |   |
    360.774522 |   1)   0.541 us    |                                          }
    360.774522 |   1)   4.663 us    |                                        }
    360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
    360.774524 |   1)   6.796 us    |                                      }
    360.774524 |   1)   7.952 us    |                                    }
    360.774525 |   1)   9.063 us    |                                  }
    360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
    360.774527 |   1)   0.578 us    |                                  __brelse();
    360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
    360.774528 |   1)               |                                    unlock_buffer() {
    360.774529 |   1)               |                                      wake_up_bit() {
    360.774529 |   1)               |                                        bit_waitqueue() {
    360.774530 |   1)   0.594 us    |                                          __phys_addr();


The function name is always displayed after the closing bracket
for a function if the start of that function is not in the
trace buffer.

Display of the function name after the closing bracket may be
enabled for functions whose start is in the trace buffer,
allowing easier searching with grep for function durations.
It is default disabled.

	- hide: echo nofuncgraph-tail > trace_options
	- show: echo funcgraph-tail > trace_options

  Example with nofuncgraph-tail (default)::

    0)               |      putname() {
    0)               |        kmem_cache_free() {
    0)   0.518 us    |          __phys_addr();
    0)   1.757 us    |        }
    0)   2.861 us    |      }

  Example with funcgraph-tail::

    0)               |      putname() {
    0)               |        kmem_cache_free() {
    0)   0.518 us    |          __phys_addr();
    0)   1.757 us    |        } /* kmem_cache_free() */
    0)   2.861 us    |      } /* putname() */

The return value of each traced function can be displayed after
an equal sign "=". When encountering system call failures, it
can be very helpful to quickly locate the function that first
returns an error code.

	- hide: echo nofuncgraph-retval > trace_options
	- show: echo funcgraph-retval > trace_options

  Example with funcgraph-retval::

    1)               |    cgroup_migrate() {
    1)   0.651 us    |      cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
    1)               |      cgroup_migrate_execute() {
    1)               |        cpu_cgroup_can_attach() {
    1)               |          cgroup_taskset_first() {
    1)   0.732 us    |            cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
    1)   1.232 us    |          } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
    1)   0.380 us    |          sched_rt_can_attach(); /* = 0x0 */
    1)   2.335 us    |        } /* cpu_cgroup_can_attach = -22 */
    1)   4.369 us    |      } /* cgroup_migrate_execute = -22 */
    1)   7.143 us    |    } /* cgroup_migrate = -22 */

The above example shows that the function cpu_cgroup_can_attach
returned the error code -22 firstly, then we can read the code
of this function to get the root cause.

When the option funcgraph-retval-hex is not set, the return value can
be displayed in a smart way. Specifically, if it is an error code,
it will be printed in signed decimal format, otherwise it will
printed in hexadecimal format.

	- smart: echo nofuncgraph-retval-hex > trace_options
	- hexadecimal: echo funcgraph-retval-hex > trace_options

  Example with funcgraph-retval-hex::

    1)               |      cgroup_migrate() {
    1)   0.651 us    |        cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
    1)               |        cgroup_migrate_execute() {
    1)               |          cpu_cgroup_can_attach() {
    1)               |            cgroup_taskset_first() {
    1)   0.732 us    |              cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
    1)   1.232 us    |            } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
    1)   0.380 us    |            sched_rt_can_attach(); /* = 0x0 */
    1)   2.335 us    |          } /* cpu_cgroup_can_attach = 0xffffffea */
    1)   4.369 us    |        } /* cgroup_migrate_execute = 0xffffffea */
    1)   7.143 us    |      } /* cgroup_migrate = 0xffffffea */

At present, there are some limitations when using the funcgraph-retval
option, and these limitations will be eliminated in the future:

- Even if the function return type is void, a return value will still
  be printed, and you can just ignore it.

- Even if return values are stored in multiple registers, only the
  value contained in the first register will be recorded and printed.
  To illustrate, in the x86 architecture, eax and edx are used to store
  a 64-bit return value, with the lower 32 bits saved in eax and the
  upper 32 bits saved in edx. However, only the value stored in eax
  will be recorded and printed.

- In certain procedure call standards, such as arm64's AAPCS64, when a
  type is smaller than a GPR, it is the responsibility of the consumer
  to perform the narrowing, and the upper bits may contain UNKNOWN values.
  Therefore, it is advisable to check the code for such cases. For instance,
  when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
  especially when larger types are truncated, whether explicitly or implicitly.
  Here are some specific cases to illustrate this point:

  **Case One**:

  The function narrow_to_u8 is defined as follows::

	u8 narrow_to_u8(u64 val)
	{
		// implicitly truncated
		return val;
	}

  It may be compiled to::

	narrow_to_u8:
		< ... ftrace instrumentation ... >
		RET

  If you pass 0x123456789abcdef to this function and want to narrow it,
  it may be recorded as 0x123456789abcdef instead of 0xef.

  **Case Two**:

  The function error_if_not_4g_aligned is defined as follows::

	int error_if_not_4g_aligned(u64 val)
	{
		if (val & GENMASK(31, 0))
			return -EINVAL;

		return 0;
	}

  It could be compiled to::

	error_if_not_4g_aligned:
		CBNZ    w0, .Lnot_aligned
		RET			// bits [31:0] are zero, bits
					// [63:32] are UNKNOWN
	.Lnot_aligned:
		MOV    x0, #-EINVAL
		RET

  When passing 0x2_0000_0000 to it, the return value may be recorded as
  0x2_0000_0000 instead of 0.

You can put some comments on specific functions by using
trace_printk() For example, if you want to put a comment inside
the __might_sleep() function, you just have to include
<linux/ftrace.h> and call trace_printk() inside __might_sleep()::

	trace_printk("I'm a comment!\n")

will produce::

   1)               |             __might_sleep() {
   1)               |                /* I'm a comment! */
   1)   1.449 us    |             }


You might find other useful features for this tracer in the
following "dynamic ftrace" section such as tracing only specific
functions or tasks.

dynamic ftrace
--------------

If CONFIG_DYNAMIC_FTRACE is set, the system will run with
virtually no overhead when function tracing is disabled. The way
this works is the mcount function call (placed at the start of
every kernel function, produced by the -pg switch in gcc),
starts of pointing to a simple return. (Enabling FTRACE will
include the -pg switch in the compiling of the kernel.)

At compile time every C file object is run through the
recordmcount program (located in the scripts directory). This
program will parse the ELF headers in the C object to find all
the locations in the .text section that call mcount. Starting
with gcc version 4.6, the -mfentry has been added for x86, which
calls "__fentry__" instead of "mcount". Which is called before
the creation of the stack frame.

Note, not all sections are traced. They may be prevented by either
a notrace, or blocked another way and all inline functions are not
traced. Check the "available_filter_functions" file to see what functions
can be traced.

A section called "__mcount_loc" is created that holds
references to all the mcount/fentry call sites in the .text section.
The recordmcount program re-links this section back into the
original object. The final linking stage of the kernel will add all these
references into a single table.

On boot up, before SMP is initialized, the dynamic ftrace code
scans this table and updates all the locations into nops. It
also records the locations, which are added to the
available_filter_functions list.  Modules are processed as they
are loaded and before they are executed.  When a module is
unloaded, it also removes its functions from the ftrace function
list. This is automatic in the module unload code, and the
module author does not need to worry about it.

When tracing is enabled, the process of modifying the function
tracepoints is dependent on architecture. The old method is to use
kstop_machine to prevent races with the CPUs executing code being
modified (which can cause the CPU to do undesirable things, especially
if the modified code crosses cache (or page) boundaries), and the nops are
patched back to calls. But this time, they do not call mcount
(which is just a function stub). They now call into the ftrace
infrastructure.

The new method of modifying the function tracepoints is to place
a breakpoint at the location to be modified, sync all CPUs, modify
the rest of the instruction not covered by the breakpoint. Sync
all CPUs again, and then remove the breakpoint with the finished
version to the ftrace call site.

Some archs do not even need to monkey around with the synchronization,
and can just slap the new code on top of the old without any
problems with other CPUs executing it at the same time.

One special side-effect to the recording of the functions being
traced is that we can now selectively choose which functions we
wish to trace and which ones we want the mcount calls to remain
as nops.

Two files are used, one for enabling and one for disabling the
tracing of specified functions. They are:

  set_ftrace_filter

and

  set_ftrace_notrace

A list of available functions that you can add to these files is
listed in:

   available_filter_functions

::

  # cat available_filter_functions
  put_prev_task_idle
  kmem_cache_create
  pick_next_task_rt
  cpus_read_lock
  pick_next_task_fair
  mutex_lock
  [...]

If I am only interested in sys_nanosleep and hrtimer_interrupt::

  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
  # echo function > current_tracer
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: function
  #
  # entries-in-buffer/entries-written: 5/5   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
            <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
            usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
            <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
            <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt

To see which functions are being traced, you can cat the file:
::

  # cat set_ftrace_filter
  hrtimer_interrupt
  sys_nanosleep


Perhaps this is not enough. The filters also allow glob(7) matching.

  ``<match>*``
	will match functions that begin with <match>
  ``*<match>``
	will match functions that end with <match>
  ``*<match>*``
	will match functions that have <match> in it
  ``<match1>*<match2>``
	will match functions that begin with <match1> and end with <match2>

.. note::
      It is better to use quotes to enclose the wild cards,
      otherwise the shell may expand the parameters into names
      of files in the local directory.

::

  # echo 'hrtimer_*' > set_ftrace_filter

Produces::

  # tracer: function
  #
  # entries-in-buffer/entries-written: 897/897   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
            <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
            <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
            <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
            <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
            <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
            <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
            <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem

Notice that we lost the sys_nanosleep.
::

  # cat set_ftrace_filter
  hrtimer_run_queues
  hrtimer_run_pending
  hrtimer_init
  hrtimer_cancel
  hrtimer_try_to_cancel
  hrtimer_forward
  hrtimer_start
  hrtimer_reprogram
  hrtimer_force_reprogram
  hrtimer_get_next_event
  hrtimer_interrupt
  hrtimer_nanosleep
  hrtimer_wakeup
  hrtimer_get_remaining
  hrtimer_get_res
  hrtimer_init_sleeper


This is because the '>' and '>>' act just like they do in bash.
To rewrite the filters, use '>'
To append to the filters, use '>>'

To clear out a filter so that all functions will be recorded
again::

 # echo > set_ftrace_filter
 # cat set_ftrace_filter
 #

Again, now we want to append.

::

  # echo sys_nanosleep > set_ftrace_filter
  # cat set_ftrace_filter
  sys_nanosleep
  # echo 'hrtimer_*' >> set_ftrace_filter
  # cat set_ftrace_filter
  hrtimer_run_queues
  hrtimer_run_pending
  hrtimer_init
  hrtimer_cancel
  hrtimer_try_to_cancel
  hrtimer_forward
  hrtimer_start
  hrtimer_reprogram
  hrtimer_force_reprogram
  hrtimer_get_next_event
  hrtimer_interrupt
  sys_nanosleep
  hrtimer_nanosleep
  hrtimer_wakeup
  hrtimer_get_remaining
  hrtimer_get_res
  hrtimer_init_sleeper


The set_ftrace_notrace prevents those functions from being
traced.
::

  # echo '*preempt*' '*lock*' > set_ftrace_notrace

Produces::

  # tracer: function
  #
  # entries-in-buffer/entries-written: 39608/39608   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
              bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
              bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
              bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
              bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
              bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
              bash-1994  [000] ....  4342.324899: do_truncate <-do_last
              bash-1994  [000] ....  4342.324899: setattr_should_drop_suidgid <-do_truncate
              bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
              bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
              bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
              bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time

We can see that there's no more lock or preempt tracing.

Selecting function filters via index
------------------------------------

Because processing of strings is expensive (the address of the function
needs to be looked up before comparing to the string being passed in),
an index can be used as well to enable functions. This is useful in the
case of setting thousands of specific functions at a time. By passing
in a list of numbers, no string processing will occur. Instead, the function
at the specific location in the internal array (which corresponds to the
functions in the "available_filter_functions" file), is selected.

::

  # echo 1 > set_ftrace_filter

Will select the first function listed in "available_filter_functions"

::

  # head -1 available_filter_functions
  trace_initcall_finish_cb

  # cat set_ftrace_filter
  trace_initcall_finish_cb

  # head -50 available_filter_functions | tail -1
  x86_pmu_commit_txn

  # echo 1 50 > set_ftrace_filter
  # cat set_ftrace_filter
  trace_initcall_finish_cb
  x86_pmu_commit_txn

Dynamic ftrace with the function graph tracer
---------------------------------------------

Although what has been explained above concerns both the
function tracer and the function-graph-tracer, there are some
special features only available in the function-graph tracer.

If you want to trace only one function and all of its children,
you just have to echo its name into set_graph_function::

 echo __do_fault > set_graph_function

will produce the following "expanded" trace of the __do_fault()
function::

   0)               |  __do_fault() {
   0)               |    filemap_fault() {
   0)               |      find_lock_page() {
   0)   0.804 us    |        find_get_page();
   0)               |        __might_sleep() {
   0)   1.329 us    |        }
   0)   3.904 us    |      }
   0)   4.979 us    |    }
   0)   0.653 us    |    _spin_lock();
   0)   0.578 us    |    page_add_file_rmap();
   0)   0.525 us    |    native_set_pte_at();
   0)   0.585 us    |    _spin_unlock();
   0)               |    unlock_page() {
   0)   0.541 us    |      page_waitqueue();
   0)   0.639 us    |      __wake_up_bit();
   0)   2.786 us    |    }
   0) + 14.237 us   |  }
   0)               |  __do_fault() {
   0)               |    filemap_fault() {
   0)               |      find_lock_page() {
   0)   0.698 us    |        find_get_page();
   0)               |        __might_sleep() {
   0)   1.412 us    |        }
   0)   3.950 us    |      }
   0)   5.098 us    |    }
   0)   0.631 us    |    _spin_lock();
   0)   0.571 us    |    page_add_file_rmap();
   0)   0.526 us    |    native_set_pte_at();
   0)   0.586 us    |    _spin_unlock();
   0)               |    unlock_page() {
   0)   0.533 us    |      page_waitqueue();
   0)   0.638 us    |      __wake_up_bit();
   0)   2.793 us    |    }
   0) + 14.012 us   |  }

You can also expand several functions at once::

 echo sys_open > set_graph_function
 echo sys_close >> set_graph_function

Now if you want to go back to trace all functions you can clear
this special filter via::

 echo > set_graph_function


ftrace_enabled
--------------

Note, the proc sysctl ftrace_enable is a big on/off switch for the
function tracer. By default it is enabled (when function tracing is
enabled in the kernel). If it is disabled, all function tracing is
disabled. This includes not only the function tracers for ftrace, but
also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
registered.

Please disable this with care.

This can be disable (and enabled) with::

  sysctl kernel.ftrace_enabled=0
  sysctl kernel.ftrace_enabled=1

 or

  echo 0 > /proc/sys/kernel/ftrace_enabled
  echo 1 > /proc/sys/kernel/ftrace_enabled


Filter commands
---------------

A few commands are supported by the set_ftrace_filter interface.
Trace commands have the following format::

  <function>:<command>:<parameter>

The following commands are supported:

- mod:
  This command enables function filtering per module. The
  parameter defines the module. For example, if only the write*
  functions in the ext3 module are desired, run:

   echo 'write*:mod:ext3' > set_ftrace_filter

  This command interacts with the filter in the same way as
  filtering based on function names. Thus, adding more functions
  in a different module is accomplished by appending (>>) to the
  filter file. Remove specific module functions by prepending
  '!'::

   echo '!writeback*:mod:ext3' >> set_ftrace_filter

  Mod command supports module globbing. Disable tracing for all
  functions except a specific module::

   echo '!*:mod:!ext3' >> set_ftrace_filter

  Disable tracing for all modules, but still trace kernel::

   echo '!*:mod:*' >> set_ftrace_filter

  Enable filter only for kernel::

   echo '*write*:mod:!*' >> set_ftrace_filter

  Enable filter for module globbing::

   echo '*write*:mod:*snd*' >> set_ftrace_filter

- traceon/traceoff:
  These commands turn tracing on and off when the specified
  functions are hit. The parameter determines how many times the
  tracing system is turned on and off. If unspecified, there is
  no limit. For example, to disable tracing when a schedule bug
  is hit the first 5 times, run::

   echo '__schedule_bug:traceoff:5' > set_ftrace_filter

  To always disable tracing when __schedule_bug is hit::

   echo '__schedule_bug:traceoff' > set_ftrace_filter

  These commands are cumulative whether or not they are appended
  to set_ftrace_filter. To remove a command, prepend it by '!'
  and drop the parameter::

   echo '!__schedule_bug:traceoff:0' > set_ftrace_filter

  The above removes the traceoff command for __schedule_bug
  that have a counter. To remove commands without counters::

   echo '!__schedule_bug:traceoff' > set_ftrace_filter

- snapshot:
  Will cause a snapshot to be triggered when the function is hit.
  ::

   echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter

  To only snapshot once:
  ::

   echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter

  To remove the above commands::

   echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
   echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter

- enable_event/disable_event:
  These commands can enable or disable a trace event. Note, because
  function tracing callbacks are very sensitive, when these commands
  are registered, the trace point is activated, but disabled in
  a "soft" mode. That is, the tracepoint will be called, but
  just will not be traced. The event tracepoint stays in this mode
  as long as there's a command that triggers it.
  ::

   echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
   	 set_ftrace_filter

  The format is::

    <function>:enable_event:<system>:<event>[:count]
    <function>:disable_event:<system>:<event>[:count]

  To remove the events commands::

   echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
   	 set_ftrace_filter
   echo '!schedule:disable_event:sched:sched_switch' > \
   	 set_ftrace_filter

- dump:
  When the function is hit, it will dump the contents of the ftrace
  ring buffer to the console. This is useful if you need to debug
  something, and want to dump the trace when a certain function
  is hit. Perhaps it's a function that is called before a triple
  fault happens and does not allow you to get a regular dump.

- cpudump:
  When the function is hit, it will dump the contents of the ftrace
  ring buffer for the current CPU to the console. Unlike the "dump"
  command, it only prints out the contents of the ring buffer for the
  CPU that executed the function that triggered the dump.

- stacktrace:
  When the function is hit, a stack trace is recorded.

trace_pipe
----------

The trace_pipe outputs the same content as the trace file, but
the effect on the tracing is different. Every read from
trace_pipe is consumed. This means that subsequent reads will be
different. The trace is live.
::

  # echo function > current_tracer
  # cat trace_pipe > /tmp/trace.out &
  [1] 4153
  # echo 1 > tracing_on
  # usleep 1
  # echo 0 > tracing_on
  # cat trace
  # tracer: function
  #
  # entries-in-buffer/entries-written: 0/0   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |

  #
  # cat /tmp/trace.out
             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath


Note, reading the trace_pipe file will block until more input is
added. This is contrary to the trace file. If any process opened
the trace file for reading, it will actually disable tracing and
prevent new entries from being added. The trace_pipe file does
not have this limitation.

trace entries
-------------

Having too much or not enough data can be troublesome in
diagnosing an issue in the kernel. The file buffer_size_kb is
used to modify the size of the internal trace buffers. The
number listed is the number of entries that can be recorded per
CPU. To know the full size, multiply the number of possible CPUs
with the number of entries.
::

  # cat buffer_size_kb
  1408 (units kilobytes)

Or simply read buffer_total_size_kb
::

  # cat buffer_total_size_kb 
  5632

To modify the buffer, simple echo in a number (in 1024 byte segments).
::

  # echo 10000 > buffer_size_kb
  # cat buffer_size_kb
  10000 (units kilobytes)

It will try to allocate as much as possible. If you allocate too
much, it can cause Out-Of-Memory to trigger.
::

  # echo 1000000000000 > buffer_size_kb
  -bash: echo: write error: Cannot allocate memory
  # cat buffer_size_kb
  85

The per_cpu buffers can be changed individually as well:
::

  # echo 10000 > per_cpu/cpu0/buffer_size_kb
  # echo 100 > per_cpu/cpu1/buffer_size_kb

When the per_cpu buffers are not the same, the buffer_size_kb
at the top level will just show an X
::

  # cat buffer_size_kb
  X

This is where the buffer_total_size_kb is useful:
::

  # cat buffer_total_size_kb 
  12916

Writing to the top level buffer_size_kb will reset all the buffers
to be the same again.

Snapshot
--------
CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
available to all non latency tracers. (Latency tracers which
record max latency, such as "irqsoff" or "wakeup", can't use
this feature, since those are already using the snapshot
mechanism internally.)

Snapshot preserves a current trace buffer at a particular point
in time without stopping tracing. Ftrace swaps the current
buffer with a spare buffer, and tracing continues in the new
current (=previous spare) buffer.

The following tracefs files in "tracing" are related to this
feature:

  snapshot:

	This is used to take a snapshot and to read the output
	of the snapshot. Echo 1 into this file to allocate a
	spare buffer and to take a snapshot (swap), then read
	the snapshot from this file in the same format as
	"trace" (described above in the section "The File
	System"). Both reads snapshot and tracing are executable
	in parallel. When the spare buffer is allocated, echoing
	0 frees it, and echoing else (positive) values clear the
	snapshot contents.
	More details are shown in the table below.

	+--------------+------------+------------+------------+
	|status\\input |     0      |     1      |    else    |
	+==============+============+============+============+
	|not allocated |(do nothing)| alloc+swap |(do nothing)|
	+--------------+------------+------------+------------+
	|allocated     |    free    |    swap    |   clear    |
	+--------------+------------+------------+------------+

Here is an example of using the snapshot feature.
::

  # echo 1 > events/sched/enable
  # echo 1 > snapshot
  # cat snapshot
  # tracer: nop
  #
  # entries-in-buffer/entries-written: 71/71   #P:8
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120   prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
             sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120   prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
  [...]
          <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120  

  # cat trace  
  # tracer: nop
  #
  # entries-in-buffer/entries-written: 77/77   #P:8
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
            <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
   snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
  [...]


If you try to use this snapshot feature when current tracer is
one of the latency tracers, you will get the following results.
::

  # echo wakeup > current_tracer
  # echo 1 > snapshot
  bash: echo: write error: Device or resource busy
  # cat snapshot
  cat: snapshot: Device or resource busy


Instances
---------
In the tracefs tracing directory, there is a directory called "instances".
This directory can have new directories created inside of it using
mkdir, and removing directories with rmdir. The directory created
with mkdir in this directory will already contain files and other
directories after it is created.
::

  # mkdir instances/foo
  # ls instances/foo
  buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
  set_event  snapshot  trace  trace_clock  trace_marker  trace_options
  trace_pipe  tracing_on

As you can see, the new directory looks similar to the tracing directory
itself. In fact, it is very similar, except that the buffer and
events are agnostic from the main directory, or from any other
instances that are created.

The files in the new directory work just like the files with the
same name in the tracing directory except the buffer that is used
is a separate and new buffer. The files affect that buffer but do not
affect the main buffer with the exception of trace_options. Currently,
the trace_options affect all instances and the top level buffer
the same, but this may change in future releases. That is, options
may become specific to the instance they reside in.

Notice that none of the function tracer files are there, nor is
current_tracer and available_tracers. This is because the buffers
can currently only have events enabled for them.
::

  # mkdir instances/foo
  # mkdir instances/bar
  # mkdir instances/zoot
  # echo 100000 > buffer_size_kb
  # echo 1000 > instances/foo/buffer_size_kb
  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
  # echo function > current_trace
  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
  # echo 1 > instances/foo/events/sched/sched_switch/enable
  # echo 1 > instances/bar/events/irq/enable
  # echo 1 > instances/zoot/events/syscalls/enable
  # cat trace_pipe
  CPU:2 [LOST 11745 EVENTS]
              bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
              bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
              bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
              bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
              bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
              bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
              bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
              bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
              bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
  [...]

  # cat instances/foo/trace_pipe
              bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
              bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
            <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
            <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
       rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
              bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
              bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
              bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
       kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
       kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
  [...]

  # cat instances/bar/trace_pipe
       migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
            <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
              bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
              bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
              bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
              bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
              bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
              bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
              sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
              sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
              sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
              sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
  [...]

  # cat instances/zoot/trace
  # tracer: nop
  #
  # entries-in-buffer/entries-written: 18996/18996   #P:4
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
              bash-1998  [000] d...   140.733501: sys_write -> 0x2
              bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
              bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
              bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
              bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
              bash-1998  [000] d...   140.733510: sys_close(fd: a)
              bash-1998  [000] d...   140.733510: sys_close -> 0x0
              bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
              bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
              bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
              bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0

You can see that the trace of the top most trace buffer shows only
the function tracing. The foo instance displays wakeups and task
switches.

To remove the instances, simply delete their directories:
::

  # rmdir instances/foo
  # rmdir instances/bar
  # rmdir instances/zoot

Note, if a process has a trace file open in one of the instance
directories, the rmdir will fail with EBUSY.


Stack trace
-----------
Since the kernel has a fixed sized stack, it is important not to
waste it in functions. A kernel developer must be conscious of
what they allocate on the stack. If they add too much, the system
can be in danger of a stack overflow, and corruption will occur,
usually leading to a system panic.

There are some tools that check this, usually with interrupts
periodically checking usage. But if you can perform a check
at every function call that will become very useful. As ftrace provides
a function tracer, it makes it convenient to check the stack size
at every function call. This is enabled via the stack tracer.

CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
::

 # echo 1 > /proc/sys/kernel/stack_tracer_enabled

You can also enable it from the kernel command line to trace
the stack size of the kernel during boot up, by adding "stacktrace"
to the kernel command line parameter.

After running it for a few minutes, the output looks like:
::

  # cat stack_max_size
  2928

  # cat stack_trace
          Depth    Size   Location    (18 entries)
          -----    ----   --------
    0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
    1)     2704     160   find_busiest_group+0x31/0x1f1
    2)     2544     256   load_balance+0xd9/0x662
    3)     2288      80   idle_balance+0xbb/0x130
    4)     2208     128   __schedule+0x26e/0x5b9
    5)     2080      16   schedule+0x64/0x66
    6)     2064     128   schedule_timeout+0x34/0xe0
    7)     1936     112   wait_for_common+0x97/0xf1
    8)     1824      16   wait_for_completion+0x1d/0x1f
    9)     1808     128   flush_work+0xfe/0x119
   10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
   11)     1664      48   input_available_p+0x1d/0x5c
   12)     1616      48   n_tty_poll+0x6d/0x134
   13)     1568      64   tty_poll+0x64/0x7f
   14)     1504     880   do_select+0x31e/0x511
   15)      624     400   core_sys_select+0x177/0x216
   16)      224      96   sys_select+0x91/0xb9
   17)      128     128   system_call_fastpath+0x16/0x1b

Note, if -mfentry is being used by gcc, functions get traced before
they set up the stack frame. This means that leaf level functions
are not tested by the stack tracer when -mfentry is used.

Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.

More
----
More details can be found in the source code, in the `kernel/trace/*.c` files.