gh-96143: Allow Linux perf profiler to see Python calls

Doc/c-api/init_config.rst

@@ -1155,6 +1155,20 @@ PyConfig
 
  Default: ``-1`` in Python mode, ``0`` in isolated mode.
 
+ .. c:member:: int perf_profiling
+
+ Enable compatibility mode with the perf profiler?
+
+ If non-zero, initialize the perf trampoline. See :ref:`perf_profiling`
+ for more information.
+
+ Set by :option:`-X perf <-X>` command line option and by the
+ :envvar:`PYTHONPERFSUPPORT` environment variable.
+
+ Default: ``-1``.
+
+ .. versionadded:: 3.12
+
  .. c:member:: int use_environment
 
  Use :ref:`environment variables <using-on-envvars>`?

Doc/howto/index.rst

@@ -30,6 +30,7 @@ Currently, the HOWTOs are:
  ipaddress.rst
  clinic.rst
  instrumentation.rst
+ perf_profiling.rst
  annotations.rst
  isolating-extensions.rst
 

Doc/howto/perf_profiling.rst

@@ -0,0 +1,200 @@
+.. highlight:: shell-session
+
+.. _perf_profiling:
+
+==============================================
+Python support for the Linux ``perf`` profiler
+==============================================
+
+:author: Pablo Galindo
+
+The Linux ``perf`` profiler is a very powerful tool that allows you to profile and
+obtain information about the performance of your application. ``perf`` also has
+a very vibrant ecosystem of tools that aid with the analysis of the data that it
+produces.
+
+The main problem with using the ``perf`` profiler with Python applications is that
+``perf`` only allows to get information about native symbols, this is, the names of
+the functions and procedures written in C. This means that the names and file names
+of the Python functions in your code will not appear in the output of the ``perf``.
+
+Since Python 3.12, the interpreter can run in a special mode that allows Python
+functions to appear in the output of the ``perf`` profiler. When this mode is
+enabled, the interpreter will interpose a small piece of code compiled on the
+fly before the execution of every Python function and it will teach ``perf`` the
+relationship between this piece of code and the associated Python function using
+`perf map files`_.
+
+.. warning::
+
+ Support for the ``perf`` profiler is only currently available for Linux on
+ selected architectures. Check the output of the configure build step or
+ check the output of ``python -m sysconfig | grep HAVE_PERF_TRAMPOLINE``
+ to see if your system is supported.
+
+For example, consider the following script:
+
+.. code-block:: python
+
+deffoo(n):
+ result =0
+for _ inrange(n):
+ result +=1
+return result
+
+defbar(n):
+ foo(n)
+
+defbaz(n):
+ bar(n)
+
+if__name__=="__main__":
+ baz(1000000)
+
+We can run perf to sample CPU stack traces at 9999 Hertz:
+
+ $ perf record -F 9999 -g -o perf.data python my_script.py
+
+Then we can use perf report to analyze the data:
+
+.. code-block:: shell-session
+
+ $ perf report --stdio -n -g
+
+ # Children Self Samples Command Shared Object Symbol
+ # ........ ........ ............ .......... .................. ..........................................
+ #
+ 91.08% 0.00% 0 python.exe python.exe [.] _start
+ |
+ ---_start
+ |
+ --90.71%--__libc_start_main
+ Py_BytesMain
+ |
+ |--56.88%--pymain_run_python.constprop.0
+ | |
+ | |--56.13%--_PyRun_AnyFileObject
+ | | _PyRun_SimpleFileObject
+ | | |
+ | | |--55.02%--run_mod
+ | | | |
+ | | | --54.65%--PyEval_EvalCode
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | |
+ | | | |--51.67%--_PyEval_EvalFrameDefault
+ | | | | |
+ | | | | |--11.52%--_PyLong_Add
+ | | | | | |
+ | | | | | |--2.97%--_PyObject_Malloc
+ ...
+
+As you can see here, the Python functions are not shown in the output, only ``_Py_Eval_EvalFrameDefault`` appears
+(the function that evaluates the Python bytecode) shows up. Unfortunately that's not very useful because all Python
+functions use the same C function to evaluate bytecode so we cannot know which Python function corresponds to which
+bytecode-evaluating function.
+
+Instead, if we run the same experiment with perf support activated we get:
+
+.. code-block:: shell-session
+
+ $ perf report --stdio -n -g
+
+ # Children Self Samples Command Shared Object Symbol
+ # ........ ........ ............ .......... .................. .....................................................................
+ #
+ 90.58% 0.36% 1 python.exe python.exe [.] _start
+ |
+ ---_start
+ |
+ --89.86%--__libc_start_main
+ Py_BytesMain
+ |
+ |--55.43%--pymain_run_python.constprop.0
+ | |
+ | |--54.71%--_PyRun_AnyFileObject
+ | | _PyRun_SimpleFileObject
+ | | |
+ | | |--53.62%--run_mod
+ | | | |
+ | | | --53.26%--PyEval_EvalCode
+ | | | py::<module>:/src/script.py
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | py::baz:/src/script.py
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | py::bar:/src/script.py
+ | | | _PyEval_EvalFrameDefault
+ | | | PyObject_Vectorcall
+ | | | _PyEval_Vector
+ | | | py::foo:/src/script.py
+ | | | |
+ | | | |--51.81%--_PyEval_EvalFrameDefault
+ | | | | |
+ | | | | |--13.77%--_PyLong_Add
+ | | | | | |
+ | | | | | |--3.26%--_PyObject_Malloc
+
+
+
+Enabling perf profiling mode
+----------------------------
+
+There are two main ways to activate the perf profiling mode. If you want it to be
+active since the start of the Python interpreter, you can use the `-Xperf` option:
+
+ $ python -Xperf my_script.py
+
+There is also support for dynamically activating and deactivating the perf
+profiling mode by using the APIs in the :mod:`sys` module:
+
+.. code-block:: python
+
+import sys
+ sys.activate_stack_trampoline("perf")
+
+# Run some code with Perf profiling active
+
+ sys.deactivate_stack_trampoline()
+
+# Perf profiling is not active anymore
+
+These APIs can be handy if you want to activate/deactivate profiling mode in
+response to a signal or other communication mechanism with your process.
+
+
+
+Now we can analyze the data with ``perf report``:
+
+ $ perf report -g -i perf.data
+
+
+How to obtain the best results
+-------------------------------
+
+For the best results, Python should be compiled with
+``CFLAGS="-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer"`` as this allows
+profilers to unwind using only the frame pointer and not on DWARF debug
+information. This is because as the code that is interposed to allow perf
+support is dynamically generated it doesn't have any DWARF debugging information
+available.
+
+You can check if you system has been compiled with this flag by running:
+
+ $ python -m sysconfig | grep 'no-omit-frame-pointer'
+
+If you don't see any output it means that your interpreter has not been compiled with
+frame pointers and therefore it may not be able to show Python functions in the output
+of ``perf``.
+
+.. _perf map files: https://github.com/torvalds/linux/blob/0513e464f9007b70b96740271a948ca5ab6e7dd7/tools/perf/Documentation/jit-interface.txt

Doc/using/cmdline.rst

@@ -535,6 +535,12 @@ Miscellaneous options
  development (running from the source tree) then the default is "off".
  Note that the "importlib_bootstrap" and "importlib_bootstrap_external"
  frozen modules are always used, even if this flag is set to "off".
+ * ``-X perf`` to activate compatibility mode with the ``perf`` profiler.
+ When this option is activated, the Linux ``perf`` profiler will be able to
+ report Python calls. This option is only available on some platforms and
+ will do nothing if is not supported on the current system. The default value
+ is "off". See also :envvar:`PYTHONPERFSUPPORT` and :ref:`perf_profiling`
+ for more information.
 
  It also allows passing arbitrary values and retrieving them through the
  :data:`sys._xoptions` dictionary.
@@ -1025,6 +1031,13 @@ conflict.
 
  .. versionadded:: 3.11
 
+.. envvar:: PYTHONPERFSUPPORT
+
+ If this variable is set to a nonzero value, it activates compatibility mode
+ with the ``perf`` profiler so Python calls can be detected by it. See the
+ :ref:`perf_profiling` section for more information.
+
+ .. versionadded:: 3.12
 
 
 Debug-mode variables

Include/cpython/initconfig.h

@@ -142,6 +142,7 @@ typedef struct PyConfig{
  unsigned long hash_seed;
  int faulthandler;
  int tracemalloc;
+ int perf_profiling;
  int import_time;
  int code_debug_ranges;
  int show_ref_count;

Include/internal/pycore_ceval.h

@@ -65,6 +65,27 @@ extern PyObject* _PyEval_BuiltinsFromGlobals(
  PyThreadState *tstate,
  PyObject *globals);
 
+// Trampoline API
+
+typedef struct{
+ // Callback to initialize the trampoline state
+ void* (*init_state)(void);
+ // Callback to register every trampoline being created
+ void (*write_state)(void* state, const void *code_addr,
+ unsigned int code_size, PyCodeObject* code);
+ // Callback to free the trampoline state
+ int (*free_state)(void* state);
+} _PyPerf_Callbacks;
+
+extern int _PyPerfTrampoline_SetCallbacks(_PyPerf_Callbacks *);
+extern void _PyPerfTrampoline_GetCallbacks(_PyPerf_Callbacks *);
+extern int _PyPerfTrampoline_Init(int activate);
+extern int _PyPerfTrampoline_Fini(void);
+extern int _PyIsPerfTrampolineActive(void);
+extern PyStatus _PyPerfTrampoline_AfterFork_Child(void);
+#ifdef PY_HAVE_PERF_TRAMPOLINE
+extern _PyPerf_Callbacks _Py_perfmap_callbacks;
+#endif
 
 static inline PyObject*
 _PyEval_EvalFrame(PyThreadState *tstate, struct _PyInterpreterFrame *frame, int throwflag)

Lib/test/test_embed.py

@@ -436,6 +436,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
  'hash_seed': 0,
  'faulthandler': 0,
  'tracemalloc': 0,
+ 'perf_profiling': 0,
  'import_time': 0,
  'code_debug_ranges': 1,
  'show_ref_count': 0,
@@ -520,6 +521,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
  use_hash_seed=0,
  faulthandler=0,
  tracemalloc=0,
+ perf_profiling=0,
  pathconfig_warnings=0,
  )
  if MS_WINDOWS:
@@ -828,6 +830,7 @@ def test_init_from_config(self):
  'use_hash_seed': 1,
  'hash_seed': 123,
  'tracemalloc': 2,
+ 'perf_profiling': 0,
  'import_time': 1,
  'code_debug_ranges': 0,
  'show_ref_count': 1,
@@ -890,6 +893,7 @@ def test_init_compat_env(self):
  'use_hash_seed': 1,
  'hash_seed': 42,
  'tracemalloc': 2,
+ 'perf_profiling': 0,
  'import_time': 1,
  'code_debug_ranges': 0,
  'malloc_stats': 1,
@@ -921,6 +925,7 @@ def test_init_python_env(self):
  'use_hash_seed': 1,
  'hash_seed': 42,
  'tracemalloc': 2,
+ 'perf_profiling': 0,
  'import_time': 1,
  'code_debug_ranges': 0,
  'malloc_stats': 1,