
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:

   * filter which processes are to be observed

   * gather data from the processes of interest

   * generate reports on the data

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-dtrace":

   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 --enable-shared), 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:

   * time in microseconds since start of script

   * name of executable

   * PID of process

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.


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;
   }
