From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/python~3.12/howto%2Finstrumentation.html | 221 +++++++++++++++++++++++ 1 file changed, 221 insertions(+) create mode 100644 devdocs/python~3.12/howto%2Finstrumentation.html (limited to 'devdocs/python~3.12/howto%2Finstrumentation.html') diff --git a/devdocs/python~3.12/howto%2Finstrumentation.html b/devdocs/python~3.12/howto%2Finstrumentation.html new file mode 100644 index 00000000..6b66b2f1 --- /dev/null +++ b/devdocs/python~3.12/howto%2Finstrumentation.html @@ -0,0 +1,221 @@ +

Instrumenting CPython with DTrace and SystemTap

author
+

David Malcolm

author
+

Łukasz Langa

DTrace and SystemTap are monitoring tools, each providing a way to inspect what the processes on a computer system are doing. They both use domain-specific languages allowing a user to write scripts which:

As of Python 3.6, CPython can be built with embedded “markers”, also known as “probes”, that can be observed by a DTrace or SystemTap script, making it easier to monitor what the CPython processes on a system are doing.

CPython implementation detail: DTrace markers are implementation details of the CPython interpreter. No guarantees are made about probe compatibility between versions of CPython. DTrace scripts can stop working or work incorrectly without warning when changing CPython versions.

Enabling the static markers

macOS comes with built-in support for DTrace. On Linux, in order to build CPython with the embedded markers for SystemTap, the SystemTap development tools must be installed.

On a Linux machine, this can be done via:

$ yum install systemtap-sdt-devel
+

or:

$ sudo apt-get install systemtap-sdt-dev
+

CPython must then be configured with the --with-dtrace option:

checking for --with-dtrace... yes
+

On macOS, you can list available DTrace probes by running a Python process in the background and listing all probes made available by the Python provider:

$ python3.6 -q &
+$ sudo dtrace -l -P python$!  # or: dtrace -l -m python3.6
+
+   ID   PROVIDER            MODULE                          FUNCTION NAME
+29564 python18035        python3.6          _PyEval_EvalFrameDefault function-entry
+29565 python18035        python3.6             dtrace_function_entry function-entry
+29566 python18035        python3.6          _PyEval_EvalFrameDefault function-return
+29567 python18035        python3.6            dtrace_function_return function-return
+29568 python18035        python3.6                           collect gc-done
+29569 python18035        python3.6                           collect gc-start
+29570 python18035        python3.6          _PyEval_EvalFrameDefault line
+29571 python18035        python3.6                 maybe_dtrace_line line
+

On Linux, you can verify if the SystemTap static markers are present in the built binary by seeing if it contains a “.note.stapsdt” section.

$ readelf -S ./python | grep .note.stapsdt
+[30] .note.stapsdt        NOTE         0000000000000000 00308d78
+

If you’ve built Python as a shared library (with the --enable-shared configure option), you need to look instead within the shared library. For example:

$ readelf -S libpython3.3dm.so.1.0 | grep .note.stapsdt
+[29] .note.stapsdt        NOTE         0000000000000000 00365b68
+

Sufficiently modern readelf can print the metadata:

$ readelf -n ./python
+
+Displaying notes found at file offset 0x00000254 with length 0x00000020:
+    Owner                 Data size          Description
+    GNU                  0x00000010          NT_GNU_ABI_TAG (ABI version tag)
+        OS: Linux, ABI: 2.6.32
+
+Displaying notes found at file offset 0x00000274 with length 0x00000024:
+    Owner                 Data size          Description
+    GNU                  0x00000014          NT_GNU_BUILD_ID (unique build ID bitstring)
+        Build ID: df924a2b08a7e89f6e11251d4602022977af2670
+
+Displaying notes found at file offset 0x002d6c30 with length 0x00000144:
+    Owner                 Data size          Description
+    stapsdt              0x00000031          NT_STAPSDT (SystemTap probe descriptors)
+        Provider: python
+        Name: gc__start
+        Location: 0x00000000004371c3, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf6
+        Arguments: -4@%ebx
+    stapsdt              0x00000030          NT_STAPSDT (SystemTap probe descriptors)
+        Provider: python
+        Name: gc__done
+        Location: 0x00000000004374e1, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf8
+        Arguments: -8@%rax
+    stapsdt              0x00000045          NT_STAPSDT (SystemTap probe descriptors)
+        Provider: python
+        Name: function__entry
+        Location: 0x000000000053db6c, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6be8
+        Arguments: 8@%rbp 8@%r12 -4@%eax
+    stapsdt              0x00000046          NT_STAPSDT (SystemTap probe descriptors)
+        Provider: python
+        Name: function__return
+        Location: 0x000000000053dba8, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bea
+        Arguments: 8@%rbp 8@%r12 -4@%eax
+

The above metadata contains information for SystemTap describing how it can patch strategically placed machine code instructions to enable the tracing hooks used by a SystemTap script.

Static DTrace probes

The following example DTrace script can be used to show the call/return hierarchy of a Python script, only tracing within the invocation of a function called “start”. In other words, import-time function invocations are not going to be listed:

self int indent;
+
+python$target:::function-entry
+/copyinstr(arg1) == "start"/
+{
+        self->trace = 1;
+}
+
+python$target:::function-entry
+/self->trace/
+{
+        printf("%d\t%*s:", timestamp, 15, probename);
+        printf("%*s", self->indent, "");
+        printf("%s:%s:%d\n", basename(copyinstr(arg0)), copyinstr(arg1), arg2);
+        self->indent++;
+}
+
+python$target:::function-return
+/self->trace/
+{
+        self->indent--;
+        printf("%d\t%*s:", timestamp, 15, probename);
+        printf("%*s", self->indent, "");
+        printf("%s:%s:%d\n", basename(copyinstr(arg0)), copyinstr(arg1), arg2);
+}
+
+python$target:::function-return
+/copyinstr(arg1) == "start"/
+{
+        self->trace = 0;
+}
+

It can be invoked like this:

$ sudo dtrace -q -s call_stack.d -c "python3.6 script.py"
+

The output looks like this:

156641360502280  function-entry:call_stack.py:start:23
+156641360518804  function-entry: call_stack.py:function_1:1
+156641360532797  function-entry:  call_stack.py:function_3:9
+156641360546807 function-return:  call_stack.py:function_3:10
+156641360563367 function-return: call_stack.py:function_1:2
+156641360578365  function-entry: call_stack.py:function_2:5
+156641360591757  function-entry:  call_stack.py:function_1:1
+156641360605556  function-entry:   call_stack.py:function_3:9
+156641360617482 function-return:   call_stack.py:function_3:10
+156641360629814 function-return:  call_stack.py:function_1:2
+156641360642285 function-return: call_stack.py:function_2:6
+156641360656770  function-entry: call_stack.py:function_3:9
+156641360669707 function-return: call_stack.py:function_3:10
+156641360687853  function-entry: call_stack.py:function_4:13
+156641360700719 function-return: call_stack.py:function_4:14
+156641360719640  function-entry: call_stack.py:function_5:18
+156641360732567 function-return: call_stack.py:function_5:21
+156641360747370 function-return:call_stack.py:start:28
+

Static SystemTap markers

The low-level way to use the SystemTap integration is to use the static markers directly. This requires you to explicitly state the binary file containing them.

For example, this SystemTap script can be used to show the call/return hierarchy of a Python script:

probe process("python").mark("function__entry") {
+     filename = user_string($arg1);
+     funcname = user_string($arg2);
+     lineno = $arg3;
+
+     printf("%s => %s in %s:%d\\n",
+            thread_indent(1), funcname, filename, lineno);
+}
+
+probe process("python").mark("function__return") {
+    filename = user_string($arg1);
+    funcname = user_string($arg2);
+    lineno = $arg3;
+
+    printf("%s <= %s in %s:%d\\n",
+           thread_indent(-1), funcname, filename, lineno);
+}
+

It can be invoked like this:

$ stap \
+  show-call-hierarchy.stp \
+  -c "./python test.py"
+

The output looks like this:

11408 python(8274):        => __contains__ in Lib/_abcoll.py:362
+11414 python(8274):         => __getitem__ in Lib/os.py:425
+11418 python(8274):          => encode in Lib/os.py:490
+11424 python(8274):          <= encode in Lib/os.py:493
+11428 python(8274):         <= __getitem__ in Lib/os.py:426
+11433 python(8274):        <= __contains__ in Lib/_abcoll.py:366
+

where the columns are:

and the remainder indicates the call/return hierarchy as the script executes.

For a --enable-shared build of CPython, the markers are contained within the libpython shared library, and the probe’s dotted path needs to reflect this. For example, this line from the above example:

probe process("python").mark("function__entry") {
+

should instead read:

probe process("python").library("libpython3.6dm.so.1.0").mark("function__entry") {
+

(assuming a debug build of CPython 3.6)

Available static markers

function__entry(str filename, str funcname, int lineno) +
+

This marker indicates that execution of a Python function has begun. It is only triggered for pure-Python (bytecode) functions.

The filename, function name, and line number are provided back to the tracing script as positional arguments, which must be accessed using $arg1, $arg2, $arg3:

  • +$arg1 : (const char *) filename, accessible using user_string($arg1) +
  • +$arg2 : (const char *) function name, accessible using user_string($arg2) +
  • +$arg3 : int line number
+
function__return(str filename, str funcname, int lineno) +
+

This marker is the converse of function__entry(), and indicates that execution of a Python function has ended (either via return, or via an exception). It is only triggered for pure-Python (bytecode) functions.

The arguments are the same as for function__entry()

+
line(str filename, str funcname, int lineno) +
+

This marker indicates a Python line is about to be executed. It is the equivalent of line-by-line tracing with a Python profiler. It is not triggered within C functions.

The arguments are the same as for function__entry().

+
gc__start(int generation) +
+

Fires when the Python interpreter starts a garbage collection cycle. arg0 is the generation to scan, like gc.collect().

+
gc__done(long collected) +
+

Fires when the Python interpreter finishes a garbage collection cycle. arg0 is the number of collected objects.

+
import__find__load__start(str modulename) +
+

Fires before importlib attempts to find and load the module. arg0 is the module name.

New in version 3.7.

+
import__find__load__done(str modulename, int found) +
+

Fires after importlib’s find_and_load function is called. arg0 is the module name, arg1 indicates if module was successfully loaded.

New in version 3.7.

+
audit(str event, void *tuple) +
+

Fires when sys.audit() or PySys_Audit() is called. arg0 is the event name as C string, arg1 is a PyObject pointer to a tuple object.

New in version 3.8.

+

SystemTap Tapsets

The higher-level way to use the SystemTap integration is to use a “tapset”: SystemTap’s equivalent of a library, which hides some of the lower-level details of the static markers.

Here is a tapset file, based on a non-shared build of CPython:

/*
+   Provide a higher-level wrapping around the function__entry and
+   function__return markers:
+ \*/
+probe python.function.entry = process("python").mark("function__entry")
+{
+    filename = user_string($arg1);
+    funcname = user_string($arg2);
+    lineno = $arg3;
+    frameptr = $arg4
+}
+probe python.function.return = process("python").mark("function__return")
+{
+    filename = user_string($arg1);
+    funcname = user_string($arg2);
+    lineno = $arg3;
+    frameptr = $arg4
+}
+

If this file is installed in SystemTap’s tapset directory (e.g. /usr/share/systemtap/tapset), then these additional probepoints become available:

python.function.entry(str filename, str funcname, int lineno, frameptr) +
+

This probe point indicates that execution of a Python function has begun. It is only triggered for pure-Python (bytecode) functions.

+
python.function.return(str filename, str funcname, int lineno, frameptr) +
+

This probe point is the converse of python.function.return, and indicates that execution of a Python function has ended (either via return, or via an exception). It is only triggered for pure-Python (bytecode) functions.

+

Examples

This SystemTap script uses the tapset above to more cleanly implement the example given above of tracing the Python function-call hierarchy, without needing to directly name the static markers:

probe python.function.entry
+{
+  printf("%s => %s in %s:%d\n",
+         thread_indent(1), funcname, filename, lineno);
+}
+
+probe python.function.return
+{
+  printf("%s <= %s in %s:%d\n",
+         thread_indent(-1), funcname, filename, lineno);
+}
+

The following script uses the tapset above to provide a top-like view of all running CPython code, showing the top 20 most frequently entered bytecode frames, each second, across the whole system:

global fn_calls;
+
+probe python.function.entry
+{
+    fn_calls[pid(), filename, funcname, lineno] += 1;
+}
+
+probe timer.ms(1000) {
+    printf("\033[2J\033[1;1H") /* clear screen \*/
+    printf("%6s %80s %6s %30s %6s\n",
+           "PID", "FILENAME", "LINE", "FUNCTION", "CALLS")
+    foreach ([pid, filename, funcname, lineno] in fn_calls- limit 20) {
+        printf("%6d %80s %6d %30s %6d\n",
+            pid, filename, lineno, funcname,
+            fn_calls[pid, filename, funcname, lineno]);
+    }
+    delete fn_calls;
+}
+
+

+ © 2001–2023 Python Software Foundation
Licensed under the PSF License.
+ https://docs.python.org/3.12/howto/instrumentation.html +

+
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