linux/kernel/trace
Steven Rostedt (Google) 44dc5c41b5 tracing: Fix wasted memory in saved_cmdlines logic
While looking at improving the saved_cmdlines cache I found a huge amount
of wasted memory that should be used for the cmdlines.

The tracing data saves pids during the trace. At sched switch, if a trace
occurred, it will save the comm of the task that did the trace. This is
saved in a "cache" that maps pids to comms and exposed to user space via
the /sys/kernel/tracing/saved_cmdlines file. Currently it only caches by
default 128 comms.

The structure that uses this creates an array to store the pids using
PID_MAX_DEFAULT (which is usually set to 32768). This causes the structure
to be of the size of 131104 bytes on 64 bit machines.

In hex: 131104 = 0x20020, and since the kernel allocates generic memory in
powers of two, the kernel would allocate 0x40000 or 262144 bytes to store
this structure. That leaves 131040 bytes of wasted space.

Worse, the structure points to an allocated array to store the comm names,
which is 16 bytes times the amount of names to save (currently 128), which
is 2048 bytes. Instead of allocating a separate array, make the structure
end with a variable length string and use the extra space for that.

This is similar to a recommendation that Linus had made about eventfs_inode names:

  https://lore.kernel.org/all/20240130190355.11486-5-torvalds@linux-foundation.org/

Instead of allocating a separate string array to hold the saved comms,
have the structure end with: char saved_cmdlines[]; and round up to the
next power of two over sizeof(struct saved_cmdline_buffers) + num_cmdlines * TASK_COMM_LEN
It will use this extra space for the saved_cmdline portion.

Now, instead of saving only 128 comms by default, by using this wasted
space at the end of the structure it can save over 8000 comms and even
saves space by removing the need for allocating the other array.

Link: https://lore.kernel.org/linux-trace-kernel/20240209063622.1f7b6d5f@rorschach.local.home

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Mete Durlu <meted@linux.ibm.com>
Fixes: 939c7a4f04 ("tracing: Introduce saved_cmdlines_size file")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
2024-02-09 06:43:21 -05:00
..
rv tracing/tools: Updates for 6.4 2023-04-28 16:11:26 -07:00
blktrace.c block: remove more NULL checks after bdev_get_queue() 2023-02-21 09:23:22 -07:00
bpf_trace.c Revert BPF token-related functionality 2023-12-19 08:23:03 -08:00
bpf_trace.h
error_report-traces.c
fgraph.c tracing: arm64: Avoid missing-prototype warnings 2023-07-12 12:06:04 -04:00
fprobe.c Probes updates for v6.7: 2023-11-01 16:15:42 -10:00
ftrace_internal.h tracing: arm64: Avoid missing-prototype warnings 2023-07-12 12:06:04 -04:00
ftrace.c ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default 2024-02-09 04:58:22 -05:00
Kconfig Probes updates for v6.5: 2023-06-30 10:44:53 -07:00
kprobe_event_gen_test.c tracing: Fix wrong return in kprobe_event_gen_test.c 2023-03-19 12:20:48 -04:00
Makefile tracing/probes: Move finding func-proto API and getting func-param API to trace_btf 2023-08-23 09:39:45 +09:00
pid_list.c
pid_list.h
power-traces.c
preemptirq_delay_test.c
rethook.c rethook: Use __rcu pointer for rethook::handler 2023-12-01 14:53:56 +09:00
ring_buffer_benchmark.c ring-buffer: Read and write to ring buffers with custom sub buffer size 2023-12-20 07:54:56 -05:00
ring_buffer.c ring-buffer: Clean ring_buffer_poll_wait() error return 2024-01-31 14:10:24 -05:00
rpm-traces.c
synth_event_gen_test.c tracing / synthetic: Disable events after testing in synth_event_gen_test_init() 2023-12-21 10:04:45 -05:00
trace_benchmark.c
trace_benchmark.h
trace_boot.c tracing: Allow creating instances with specified system events 2023-12-18 23:14:16 -05:00
trace_branch.c
trace_btf.c tracing/probes: Add a function to search a member of a struct/union 2023-08-23 09:40:16 +09:00
trace_btf.h tracing/probes: Add a function to search a member of a struct/union 2023-08-23 09:40:16 +09:00
trace_clock.c
trace_dynevent.c
trace_dynevent.h
trace_entries.h tracing: Add back FORTIFY_SOURCE logic to kernel_stack event structure 2023-07-30 18:11:44 -04:00
trace_eprobe.c tracing/eprobe: drop unneeded breaks 2023-10-10 01:03:48 +09:00
trace_event_perf.c
trace_events_filter_test.h
trace_events_filter.c tracing: Have trace_event_file have ref counters 2023-11-01 23:44:44 -04:00
trace_events_hist.c tracing histograms: Simplify parse_actions() function 2024-01-08 13:24:56 -05:00
trace_events_inject.c tracing: Have event inject files inc the trace array ref count 2023-09-07 16:38:54 -04:00
trace_events_synth.c tracing/synthetic: fix kernel-doc warnings 2023-12-20 12:51:03 -05:00
trace_events_trigger.c tracing/trigger: Fix to return error if failed to alloc snapshot 2024-01-26 15:10:24 -05:00
trace_events_user.c iov_iter: replace import_single_range() with import_ubuf() 2023-12-05 11:57:37 +01:00
trace_events.c ring-buffer: Page size per ring buffer 2023-12-20 07:54:55 -05:00
trace_export.c tracing: Add back FORTIFY_SOURCE logic to kernel_stack event structure 2023-07-30 18:11:44 -04:00
trace_fprobe.c tracing: fprobe-event: Fix to check tracepoint event and return 2023-11-10 20:06:12 +09:00
trace_functions_graph.c function_graph: Support recording and printing the return value of function 2023-06-20 18:38:37 -04:00
trace_functions.c
trace_hwlat.c tracing: Remove extra space at the end of hwlat_detector/mode 2023-09-01 21:00:00 -04:00
trace_irqsoff.c tracing: Fix memleak due to race between current_tracer and trace 2023-08-17 13:49:37 -04:00
trace_kdb.c
trace_kprobe_selftest.c tracing: arm64: Avoid missing-prototype warnings 2023-07-12 12:06:04 -04:00
trace_kprobe_selftest.h
trace_kprobe.c trace/kprobe: Display the actual notrace function when rejecting a probe 2023-12-17 16:28:54 +09:00
trace_mmiotrace.c
trace_nop.c
trace_osnoise.c tracing/timerlat: Move hrtimer_init to timerlat_fd open() 2024-02-01 11:50:13 -05:00
trace_output.c tracing: Add size check when printing trace_marker output 2023-12-12 22:07:22 -05:00
trace_output.h tracing: Add "fields" option to show raw trace event fields 2023-03-29 06:52:08 -04:00
trace_preemptirq.c cpuidle: tracing, preempt: Squash _rcuidle tracing 2023-01-31 15:01:46 +01:00
trace_printk.c
trace_probe_kernel.h tracing/probes: Fix to record 0-length data_loc in fetch_store_string*() if fails 2023-07-14 17:04:58 +09:00
trace_probe_tmpl.h tracing/probes: Fix to record 0-length data_loc in fetch_store_string*() if fails 2023-07-14 17:04:58 +09:00
trace_probe.c tracing/probes: Add string type check with BTF 2023-08-23 09:41:13 +09:00
trace_probe.h tracing/kprobes: Return EADDRNOTAVAIL when func matches several symbols 2023-10-20 22:10:41 +09:00
trace_recursion_record.c
trace_sched_switch.c
trace_sched_wakeup.c tracing: Fix memleak due to race between current_tracer and trace 2023-08-17 13:49:37 -04:00
trace_selftest_dynamic.c
trace_selftest.c tracing: Have function_graph selftest call cond_resched() 2023-05-28 21:15:46 -04:00
trace_seq.c trace_seq: Increase the buffer size to almost two pages 2023-12-18 23:14:16 -05:00
trace_stack.c
trace_stat.c
trace_stat.h
trace_synth.h tracing: Allow synthetic events to pass around stacktraces 2023-01-25 10:31:24 -05:00
trace_syscalls.c bpf: Change syscall_nr type to int in struct syscall_tp_t 2023-10-13 12:39:36 -07:00
trace_uprobe.c tracing/uprobe: Replace strlcpy() with strscpy() 2023-12-01 10:25:35 -08:00
trace.c tracing: Fix wasted memory in saved_cmdlines logic 2024-02-09 06:43:21 -05:00
trace.h ring-buffer: Page size per ring buffer 2023-12-20 07:54:55 -05:00
tracing_map.c tracing: Ensure visibility when inserting an element into tracing_map 2024-01-22 17:15:40 -05:00
tracing_map.h tracing: Remove unused extern declaration tracing_map_set_field_descr() 2023-07-23 11:08:14 -04:00