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gh-96143: Improve perf profiler docs (#96445) · python/cpython@723ebe7 · GitHub

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gh-96143: Improve perf profiler docs (#96445)
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‎Doc/howto/perf_profiling.rst‎

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@@ -8,10 +8,11 @@ Python support for the Linux ``perf`` profiler
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:author: Pablo Galindo
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The Linux ``perf`` profiler is a very powerful tool that allows you to profile and
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obtain information about the performance of your application. ``perf`` also has
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a very vibrant ecosystem of tools that aid with the analysis of the data that it
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produces.
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`The Linux perf profiler <https://perf.wiki.kernel.org>`_
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is a very powerful tool that allows you to profile and obtain
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information about the performance of your application.
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``perf`` also has a very vibrant ecosystem of tools
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that aid with the analysis of the data that it produces.
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The main problem with using the ``perf`` profiler with Python applications is that
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``perf`` only allows to get information about native symbols, this is, the names of
@@ -25,7 +26,7 @@ fly before the execution of every Python function and it will teach ``perf`` the
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relationship between this piece of code and the associated Python function using
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`perf map files`_.
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.. warning::
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.. note::
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Support for the ``perf`` profiler is only currently available for Linux on
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selected architectures. Check the output of the configure build step or
@@ -51,11 +52,11 @@ For example, consider the following script:
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if __name__ == "__main__":
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baz(1000000)
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We can run perf to sample CPU stack traces at 9999 Hertz:
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We can run ``perf`` to sample CPU stack traces at 9999 Hertz::
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$ perf record -F 9999 -g -o perf.data python my_script.py
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Then we can use perf report to analyze the data:
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Then we can use ``perf`` report to analyze the data:
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.. code-block:: shell-session
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@@ -101,7 +102,7 @@ As you can see here, the Python functions are not shown in the output, only ``_P
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functions use the same C function to evaluate bytecode so we cannot know which Python function corresponds to which
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bytecode-evaluating function.
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Instead, if we run the same experiment with perf support activated we get:
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Instead, if we run the same experiment with ``perf`` support enabled we get:
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.. code-block:: shell-session
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@@ -147,52 +148,58 @@ Instead, if we run the same experiment with perf support activated we get:
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Enabling perf profiling mode
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----------------------------
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How to enable ``perf`` profiling support
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----------------------------------------
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There are two main ways to activate the perf profiling mode. If you want it to be
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active since the start of the Python interpreter, you can use the ``-Xperf`` option:
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``perf`` profiling support can either be enabled from the start using
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the environment variable :envvar:`PYTHONPERFSUPPORT` or the
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:option:`-X perf <-X>` option,
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or dynamically using :func:`sys.activate_stack_trampoline` and
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:func:`sys.deactivate_stack_trampoline`.
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$ python -Xperf my_script.py
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The :mod:`!sys` functions take precedence over the :option:`!-X` option,
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the :option:`!-X` option takes precedence over the environment variable.
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You can also set the :envvar:`PYTHONPERFSUPPORT` to a nonzero value to actiavate perf
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profiling mode globally.
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Example, using the environment variable::
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There is also support for dynamically activating and deactivating the perf
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profiling mode by using the APIs in the :mod:`sys` module:
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$ PYTHONPERFSUPPORT=1
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$ python script.py
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$ perf report -g -i perf.data
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.. code-block:: python
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import sys
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sys.activate_stack_trampoline("perf")
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Example, using the :option:`!-X` option::
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# Run some code with Perf profiling active
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$ python -X perf script.py
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$ perf report -g -i perf.data
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sys.deactivate_stack_trampoline()
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Example, using the :mod:`sys` APIs in file :file:`example.py`:
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# Perf profiling is not active anymore
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.. code-block:: python
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These APIs can be handy if you want to activate/deactivate profiling mode in
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response to a signal or other communication mechanism with your process.
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import sys
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sys.activate_stack_trampoline("perf")
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do_profiled_stuff()
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sys.deactivate_stack_trampoline()
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non_profiled_stuff()
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Now we can analyze the data with ``perf report``:
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...then::
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$ perf report -g -i perf.data
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$ python ./example.py
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$ perf report -g -i perf.data
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How to obtain the best results
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-------------------------------
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------------------------------
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For the best results, Python should be compiled with
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``CFLAGS="-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer"`` as this allows
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profilers to unwind using only the frame pointer and not on DWARF debug
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information. This is because as the code that is interposed to allow perf
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information. This is because as the code that is interposed to allow ``perf``
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support is dynamically generated it doesn't have any DWARF debugging information
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available.
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You can check if you system has been compiled with this flag by running:
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You can check if your system has been compiled with this flag by running::
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$ python -m sysconfig | grep 'no-omit-frame-pointer'
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‎Doc/library/sys.rst‎

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@@ -1555,6 +1555,38 @@ always available.
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This function has been added on a provisional basis (see :pep:`411`
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for details.) Use it only for debugging purposes.
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.. function:: activate_stack_trampoline(backend, /)
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Activate the stack profiler trampoline *backend*.
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The only supported backend is ``"perf"``.
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.. availability:: Linux.
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.. versionadded:: 3.12
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.. seealso::
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* :ref:`perf_profiling`
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* https://perf.wiki.kernel.org
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.. function:: deactivate_stack_trampoline()
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Deactivate the current stack profiler trampoline backend.
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If no stack profiler is activated, this function has no effect.
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.. availability:: Linux.
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.. versionadded:: 3.12
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.. function:: is_stack_trampoline_active()
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Return ``True`` if a stack profiler trampoline is active.
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.. availability:: Linux.
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.. versionadded:: 3.12
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.. function:: _enablelegacywindowsfsencoding()
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Changes the :term:`filesystem encoding and error handler` to 'mbcs' and

‎Doc/using/cmdline.rst‎

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@@ -538,12 +538,11 @@ Miscellaneous options
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development (running from the source tree) then the default is "off".
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Note that the "importlib_bootstrap" and "importlib_bootstrap_external"
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frozen modules are always used, even if this flag is set to "off".
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* ``-X perf`` to activate compatibility mode with the ``perf`` profiler.
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When this option is activated, the Linux ``perf`` profiler will be able to
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* ``-X perf`` enables support for the Linux ``perf`` profiler.
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When this option is provided, the ``perf`` profiler will be able to
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report Python calls. This option is only available on some platforms and
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will do nothing if is not supported on the current system. The default value
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is "off". See also :envvar:`PYTHONPERFSUPPORT` and :ref:`perf_profiling`
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for more information.
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is "off". See also :envvar:`PYTHONPERFSUPPORT` and :ref:`perf_profiling`.
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It also allows passing arbitrary values and retrieving them through the
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:data:`sys._xoptions` dictionary.
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.. envvar:: PYTHONPERFSUPPORT
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If this variable is set to a nonzero value, it activates compatibility mode
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with the ``perf`` profiler so Python calls can be detected by it. See the
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:ref:`perf_profiling` section for more information.
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If this variable is set to a nonzero value, it enables support for
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the Linux ``perf`` profiler so Python calls can be detected by it.
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If set to ``0``, disable Linux ``perf`` profiler support.
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See also the :option:`-X perf <-X>` command-line option
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and :ref:`perf_profiling`.
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.. versionadded:: 3.12
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‎Doc/whatsnew/3.12.rst‎

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@@ -74,6 +74,15 @@ Important deprecations, removals or restrictions:
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New Features
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============
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* Add :ref:`perf_profiling` through the new
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environment variable :envvar:`PYTHONPERFSUPPORT`,
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the new command-line option :option:`-X perf <-X>`,
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as well as the new :func:`sys.activate_stack_trampoline`,
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:func:`sys.deactivate_stack_trampoline`,
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and :func:`sys.is_stack_trampoline_active` APIs.
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(Design by Pablo Galindo. Contributed by Pablo Galindo and Christian Heimes
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with contributions from Gregory P. Smith [Google] and Mark Shannon
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in :gh:`96123`.)
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Other Language Changes
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The :class:`tempfile.NamedTemporaryFile` function has a new optional parameter
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*delete_on_close* (Contributed by Evgeny Zorin in :gh:`58451`.)
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sys
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---
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* Add :func:`sys.activate_stack_trampoline` and
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:func:`sys.deactivate_stack_trampoline` for activating and deactivating
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stack profiler trampolines,
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and :func:`sys.is_stack_trampoline_active` for querying if stack profiler
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trampolines are active.
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(Contributed by Pablo Galindo and Christian Heimes
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with contributions from Gregory P. Smith [Google] and Mark Shannon
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in :gh:`96123`.)
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Optimizations
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=============
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‎Python/clinic/sysmodule.c.h‎

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‎Python/sysmodule.c‎

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@@ -2127,12 +2127,12 @@ sys.activate_stack_trampoline
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backend: str
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/
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Activate the perf profiler trampoline.
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Activate stack profiler trampoline *backend*.
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[clinic start generated code]*/
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static PyObject *
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sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
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/*[clinic end generated code: output=5783cdeb51874b43 input=b09020e3a17c78c5]*/
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/*[clinic end generated code: output=5783cdeb51874b43 input=a12df928758a82b4]*/
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{
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#ifdef PY_HAVE_PERF_TRAMPOLINE
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if (strcmp(backend, "perf") == 0) {
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/*[clinic input]
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sys.deactivate_stack_trampoline
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Dectivate the perf profiler trampoline.
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Deactivate the current stack profiler trampoline backend.
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If no stack profiler is activated, this function has no effect.
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[clinic start generated code]*/
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static PyObject *
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sys_deactivate_stack_trampoline_impl(PyObject *module)
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/*[clinic end generated code: output=b50da25465df0ef1 input=491f4fc1ed615736]*/
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/*[clinic end generated code: output=b50da25465df0ef1 input=9f629a6be9fe7fc8]*/
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{
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if (_PyPerfTrampoline_Init(0) < 0) {
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return NULL;
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/*[clinic input]
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sys.is_stack_trampoline_active
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Returns *True* if the perf profiler trampoline is active.
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Return *True* if a stack profiler trampoline is active.
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[clinic start generated code]*/
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static PyObject *
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sys_is_stack_trampoline_active_impl(PyObject *module)
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/*[clinic end generated code: output=ab2746de0ad9d293 input=061fa5776ac9dd59]*/
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/*[clinic end generated code: output=ab2746de0ad9d293 input=29616b7bf6a0b703]*/
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{
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#ifdef PY_HAVE_PERF_TRAMPOLINE
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if (_PyIsPerfTrampolineActive()) {

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