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409 lines
14 KiB
ReStructuredText
409 lines
14 KiB
ReStructuredText
.. highlight:: shell-session
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.. _instrumentation:
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===============================================
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Instrumenting CPython with DTrace and SystemTap
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===============================================
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:author: David Malcolm
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:author: Łukasz Langa
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DTrace and SystemTap are monitoring tools, each providing a way to inspect
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what the processes on a computer system are doing. They both use
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domain-specific languages allowing a user to write scripts which:
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- filter which processes are to be observed
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- gather data from the processes of interest
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- generate reports on the data
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As of Python 3.6, CPython can be built with embedded "markers", also
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known as "probes", that can be observed by a DTrace or SystemTap script,
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making it easier to monitor what the CPython processes on a system are
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doing.
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.. impl-detail::
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DTrace markers are implementation details of the CPython interpreter.
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No guarantees are made about probe compatibility between versions of
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CPython. DTrace scripts can stop working or work incorrectly without
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warning when changing CPython versions.
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Enabling the static markers
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---------------------------
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macOS comes with built-in support for DTrace. On Linux, in order to
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build CPython with the embedded markers for SystemTap, the SystemTap
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development tools must be installed.
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On a Linux machine, this can be done via::
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$ yum install systemtap-sdt-devel
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or::
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$ sudo apt-get install systemtap-sdt-dev
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CPython must then be configured ``--with-dtrace``:
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.. code-block:: none
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checking for --with-dtrace... yes
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On macOS, you can list available DTrace probes by running a Python
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process in the background and listing all probes made available by the
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Python provider::
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$ python3.6 -q &
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$ sudo dtrace -l -P python$! # or: dtrace -l -m python3.6
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ID PROVIDER MODULE FUNCTION NAME
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29564 python18035 python3.6 _PyEval_EvalFrameDefault function-entry
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29565 python18035 python3.6 dtrace_function_entry function-entry
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29566 python18035 python3.6 _PyEval_EvalFrameDefault function-return
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29567 python18035 python3.6 dtrace_function_return function-return
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29568 python18035 python3.6 collect gc-done
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29569 python18035 python3.6 collect gc-start
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29570 python18035 python3.6 _PyEval_EvalFrameDefault line
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29571 python18035 python3.6 maybe_dtrace_line line
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On Linux, you can verify if the SystemTap static markers are present in
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the built binary by seeing if it contains a ".note.stapsdt" section.
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::
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$ readelf -S ./python | grep .note.stapsdt
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[30] .note.stapsdt NOTE 0000000000000000 00308d78
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If you've built Python as a shared library (with --enable-shared), you
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need to look instead within the shared library. For example::
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$ readelf -S libpython3.3dm.so.1.0 | grep .note.stapsdt
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[29] .note.stapsdt NOTE 0000000000000000 00365b68
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Sufficiently modern readelf can print the metadata::
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$ readelf -n ./python
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Displaying notes found at file offset 0x00000254 with length 0x00000020:
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Owner Data size Description
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GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
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OS: Linux, ABI: 2.6.32
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Displaying notes found at file offset 0x00000274 with length 0x00000024:
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Owner Data size Description
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GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
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Build ID: df924a2b08a7e89f6e11251d4602022977af2670
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Displaying notes found at file offset 0x002d6c30 with length 0x00000144:
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Owner Data size Description
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stapsdt 0x00000031 NT_STAPSDT (SystemTap probe descriptors)
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Provider: python
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Name: gc__start
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Location: 0x00000000004371c3, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf6
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Arguments: -4@%ebx
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stapsdt 0x00000030 NT_STAPSDT (SystemTap probe descriptors)
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Provider: python
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Name: gc__done
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Location: 0x00000000004374e1, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf8
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Arguments: -8@%rax
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stapsdt 0x00000045 NT_STAPSDT (SystemTap probe descriptors)
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Provider: python
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Name: function__entry
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Location: 0x000000000053db6c, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6be8
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Arguments: 8@%rbp 8@%r12 -4@%eax
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stapsdt 0x00000046 NT_STAPSDT (SystemTap probe descriptors)
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Provider: python
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Name: function__return
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Location: 0x000000000053dba8, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bea
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Arguments: 8@%rbp 8@%r12 -4@%eax
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The above metadata contains information for SystemTap describing how it
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can patch strategically-placed machine code instructions to enable the
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tracing hooks used by a SystemTap script.
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Static DTrace probes
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--------------------
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The following example DTrace script can be used to show the call/return
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hierarchy of a Python script, only tracing within the invocation of
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a function called "start". In other words, import-time function
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invocations are not going to be listed:
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.. code-block:: none
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self int indent;
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python$target:::function-entry
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/copyinstr(arg1) == "start"/
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{
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self->trace = 1;
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}
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python$target:::function-entry
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/self->trace/
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{
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printf("%d\t%*s:", timestamp, 15, probename);
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printf("%*s", self->indent, "");
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printf("%s:%s:%d\n", basename(copyinstr(arg0)), copyinstr(arg1), arg2);
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self->indent++;
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}
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python$target:::function-return
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/self->trace/
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{
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self->indent--;
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printf("%d\t%*s:", timestamp, 15, probename);
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printf("%*s", self->indent, "");
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printf("%s:%s:%d\n", basename(copyinstr(arg0)), copyinstr(arg1), arg2);
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}
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python$target:::function-return
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/copyinstr(arg1) == "start"/
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{
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self->trace = 0;
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}
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It can be invoked like this::
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$ sudo dtrace -q -s call_stack.d -c "python3.6 script.py"
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The output looks like this:
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.. code-block:: none
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156641360502280 function-entry:call_stack.py:start:23
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156641360518804 function-entry: call_stack.py:function_1:1
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156641360532797 function-entry: call_stack.py:function_3:9
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156641360546807 function-return: call_stack.py:function_3:10
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156641360563367 function-return: call_stack.py:function_1:2
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156641360578365 function-entry: call_stack.py:function_2:5
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156641360591757 function-entry: call_stack.py:function_1:1
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156641360605556 function-entry: call_stack.py:function_3:9
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156641360617482 function-return: call_stack.py:function_3:10
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156641360629814 function-return: call_stack.py:function_1:2
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156641360642285 function-return: call_stack.py:function_2:6
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156641360656770 function-entry: call_stack.py:function_3:9
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156641360669707 function-return: call_stack.py:function_3:10
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156641360687853 function-entry: call_stack.py:function_4:13
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156641360700719 function-return: call_stack.py:function_4:14
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156641360719640 function-entry: call_stack.py:function_5:18
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156641360732567 function-return: call_stack.py:function_5:21
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156641360747370 function-return:call_stack.py:start:28
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Static SystemTap markers
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------------------------
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The low-level way to use the SystemTap integration is to use the static
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markers directly. This requires you to explicitly state the binary file
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containing them.
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For example, this SystemTap script can be used to show the call/return
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hierarchy of a Python script:
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.. code-block:: none
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probe process("python").mark("function__entry") {
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filename = user_string($arg1);
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funcname = user_string($arg2);
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lineno = $arg3;
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printf("%s => %s in %s:%d\\n",
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thread_indent(1), funcname, filename, lineno);
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}
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probe process("python").mark("function__return") {
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filename = user_string($arg1);
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funcname = user_string($arg2);
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lineno = $arg3;
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printf("%s <= %s in %s:%d\\n",
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thread_indent(-1), funcname, filename, lineno);
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}
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It can be invoked like this::
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$ stap \
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show-call-hierarchy.stp \
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-c "./python test.py"
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The output looks like this:
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.. code-block:: none
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11408 python(8274): => __contains__ in Lib/_abcoll.py:362
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11414 python(8274): => __getitem__ in Lib/os.py:425
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11418 python(8274): => encode in Lib/os.py:490
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11424 python(8274): <= encode in Lib/os.py:493
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11428 python(8274): <= __getitem__ in Lib/os.py:426
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11433 python(8274): <= __contains__ in Lib/_abcoll.py:366
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where the columns are:
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- time in microseconds since start of script
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- name of executable
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- PID of process
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and the remainder indicates the call/return hierarchy as the script executes.
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For a `--enable-shared` build of CPython, the markers are contained within the
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libpython shared library, and the probe's dotted path needs to reflect this. For
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example, this line from the above example::
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probe process("python").mark("function__entry") {
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should instead read::
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probe process("python").library("libpython3.6dm.so.1.0").mark("function__entry") {
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(assuming a debug build of CPython 3.6)
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Available static markers
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------------------------
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.. I'm reusing the "c:function" type for markers
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.. c:function:: function__entry(str filename, str funcname, int lineno)
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This marker indicates that execution of a Python function has begun.
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It is only triggered for pure-Python (bytecode) functions.
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The filename, function name, and line number are provided back to the
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tracing script as positional arguments, which must be accessed using
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``$arg1``, ``$arg2``, ``$arg3``:
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* ``$arg1`` : ``(const char *)`` filename, accessible using ``user_string($arg1)``
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* ``$arg2`` : ``(const char *)`` function name, accessible using
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``user_string($arg2)``
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* ``$arg3`` : ``int`` line number
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.. c:function:: function__return(str filename, str funcname, int lineno)
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This marker is the converse of :c:func:`function__entry`, and indicates that
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execution of a Python function has ended (either via ``return``, or via an
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exception). It is only triggered for pure-Python (bytecode) functions.
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The arguments are the same as for :c:func:`function__entry`
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.. c:function:: line(str filename, str funcname, int lineno)
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This marker indicates a Python line is about to be executed. It is
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the equivalent of line-by-line tracing with a Python profiler. It is
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not triggered within C functions.
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The arguments are the same as for :c:func:`function__entry`.
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.. c:function:: gc__start(int generation)
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Fires when the Python interpreter starts a garbage collection cycle.
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``arg0`` is the generation to scan, like :func:`gc.collect()`.
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.. c:function:: gc__done(long collected)
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Fires when the Python interpreter finishes a garbage collection
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cycle. ``arg0`` is the number of collected objects.
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SystemTap Tapsets
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-----------------
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The higher-level way to use the SystemTap integration is to use a "tapset":
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SystemTap's equivalent of a library, which hides some of the lower-level
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details of the static markers.
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Here is a tapset file, based on a non-shared build of CPython:
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.. code-block:: none
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/*
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Provide a higher-level wrapping around the function__entry and
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function__return markers:
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\*/
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probe python.function.entry = process("python").mark("function__entry")
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{
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filename = user_string($arg1);
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funcname = user_string($arg2);
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lineno = $arg3;
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frameptr = $arg4
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}
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probe python.function.return = process("python").mark("function__return")
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{
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filename = user_string($arg1);
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funcname = user_string($arg2);
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lineno = $arg3;
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frameptr = $arg4
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}
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If this file is installed in SystemTap's tapset directory (e.g.
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``/usr/share/systemtap/tapset``), then these additional probepoints become
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available:
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.. c:function:: python.function.entry(str filename, str funcname, int lineno, frameptr)
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This probe point indicates that execution of a Python function has begun.
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It is only triggered for pure-python (bytecode) functions.
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.. c:function:: python.function.return(str filename, str funcname, int lineno, frameptr)
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This probe point is the converse of :c:func:`python.function.return`, and
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indicates that execution of a Python function has ended (either via
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``return``, or via an exception). It is only triggered for pure-python
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(bytecode) functions.
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Examples
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--------
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This SystemTap script uses the tapset above to more cleanly implement the
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example given above of tracing the Python function-call hierarchy, without
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needing to directly name the static markers:
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.. code-block:: none
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probe python.function.entry
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{
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printf("%s => %s in %s:%d\n",
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thread_indent(1), funcname, filename, lineno);
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}
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probe python.function.return
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{
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printf("%s <= %s in %s:%d\n",
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thread_indent(-1), funcname, filename, lineno);
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}
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The following script uses the tapset above to provide a top-like view of all
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running CPython code, showing the top 20 most frequently-entered bytecode
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frames, each second, across the whole system:
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.. code-block:: none
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global fn_calls;
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probe python.function.entry
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{
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fn_calls[pid(), filename, funcname, lineno] += 1;
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}
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probe timer.ms(1000) {
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printf("\033[2J\033[1;1H") /* clear screen \*/
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printf("%6s %80s %6s %30s %6s\n",
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"PID", "FILENAME", "LINE", "FUNCTION", "CALLS")
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foreach ([pid, filename, funcname, lineno] in fn_calls- limit 20) {
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printf("%6d %80s %6d %30s %6d\n",
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pid, filename, lineno, funcname,
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fn_calls[pid, filename, funcname, lineno]);
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}
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delete fn_calls;
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}
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