# DTrace と SystemTap で CPython を測定する

author:

  David Malcolm

author:

  Łukasz Langa

Dtrace と SystemTap はモニタリングツールで、コンピュータシステムのプロセスが何をしているかを調べる方法を提供します。 どちらもドメイン固有言語 (domain-specific language) を使用して、次のことができるスクリプトをユーザが書けます:

- 観測対象のプロセスを絞り込む

- 関心のあるプロセスからデータを収集する

- 収集したデータからレポートを生成する

Python 3.6 では、 CPython は "プローブ" としても知られる "マーカー" を埋め込んだビルドが行えます。 マーカーは DTrace や SystemTap のスクリプトから観測でき、システムの CPython プロセスが何をしているかを観察するのが簡単になります。

DTrace マーカーは CPython インタプリタの実装詳細です。 CPython のバージョン間でプローブの互換性があるという保証はありません。 CPython のバージョンを変えると、 DTrace スクリプトは警告無しに動作しなくなったり、おかしな動作をする可能性があります。

## 静的マーカーの有効化

macOS には組み込みの DTrace サポートが備わっています。 Linux では SystemTap 用のマーカーを埋め込んで CPython をビルドするためには、 SystemTap 開発ツールをインストールしなければなりません。

Linux マシンでは、 SystemTap 開発ツールのインストールは次のように行えます:

    $ yum install systemtap-sdt-devel

もしくは:

    $ 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

十分に新しい readelf ではメタデータを出力できます:

    $ 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.

## 静的 DTrace プローブ

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;
    }

この例は次のように実行できます:

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

出力はこのようになります:

    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

## 静的 SystemTap マーカー

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.

例えば、この SystemTap スクリプトは Python の呼び出し/返却 (call/return) 階層を表示するのに使えます:

    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);
    }

この例は次のように実行できます:

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

出力はこのようになります:

    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

それぞれの列の内容は次の通りです:

- スクリプトを起動してからのマイクロ秒単位の時間

- 実行可能ファイルの名前

- プロセスの PID

そして、残りの部分はスクリプトが実行していた呼び出し/返却階層を表示しています。

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)

## 利用可能な静的マーカー

function__entry(str filename, str funcname, int lineno)

  このマーカーは Python の関数の実行が開始されたことを示しています。 このマーカーは、ピュア Python (バイトコード) の関数でしか起動されません。

  トレーススクリプトには位置引数として、ファイル名、関数名、行番号が渡され、必ず `$arg1`, `$arg2`, `$arg3` で渡されます:

  > - `$arg1` : `(const char *)` ファイル名、 `user_string($arg1)` でアクセスできます

  > - `$arg2` : `(const char *)` 関数名、 `user_string($arg2)` でアクセスできます

  > - `$arg3` : `int` 行番号

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)

  このマーカーは、これから実行される Python の行を示しています。 これは Python プロファイラによる行ごとのトレースと同等です。 このマーカーは C 関数の内部では起動されません。

  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)

  Python インタプリタによる循環参照のガベージコレクションが完了したときに発火します。 `arg0` は回収したオブジェクトの数です。

import__find__load__start(str modulename)

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

  Added 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.

  Added 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.

  Added in version 3.8.

### C Entry Points

To simplify triggering of DTrace markers, Python's C API comes with a number of helper functions that mirror each static marker. On builds of Python without DTrace enabled, these do nothing.

In general, it is not necessary to call these yourself, as Python will do it for you.

C API Function | Static Marker | 注釈
--- | --- | ---
void PyDTrace_LINE(const char *arg0, const char *arg1, int arg2) | `line()` |
void PyDTrace_FUNCTION_ENTRY(const char *arg0, const char *arg1, int arg2) | `function__entry()` |
void PyDTrace_FUNCTION_RETURN(const char *arg0, const char *arg1, int arg2) | `function__return()` |
void PyDTrace_GC_START(int arg0) | `gc__start()` |
void PyDTrace_GC_DONE(Py_ssize_t arg0) | `gc__done()` |
void PyDTrace_INSTANCE_NEW_START(int arg0) | `instance__new__start()` | Not used by Python
void PyDTrace_INSTANCE_NEW_DONE(int arg0) | `instance__new__done()` | Not used by Python
void PyDTrace_INSTANCE_DELETE_START(int arg0) | `instance__delete__start()` | Not used by Python
void PyDTrace_INSTANCE_DELETE_DONE(int arg0) | `instance__delete__done()` | Not used by Python
void PyDTrace_IMPORT_FIND_LOAD_START(const char *arg0) | `import__find__load__start()` |
void PyDTrace_IMPORT_FIND_LOAD_DONE(const char *arg0, int arg1) | `import__find__load__done()` |
void PyDTrace_AUDIT(const char *arg0, void *arg1) | `audit()` |

### C Probing Checks

int PyDTrace_LINE_ENABLED(void)

int PyDTrace_FUNCTION_ENTRY_ENABLED(void)

int PyDTrace_FUNCTION_RETURN_ENABLED(void)

int PyDTrace_GC_START_ENABLED(void)

int PyDTrace_GC_DONE_ENABLED(void)

int PyDTrace_INSTANCE_NEW_START_ENABLED(void)

int PyDTrace_INSTANCE_NEW_DONE_ENABLED(void)

int PyDTrace_INSTANCE_DELETE_START_ENABLED(void)

int PyDTrace_INSTANCE_DELETE_DONE_ENABLED(void)

int PyDTrace_IMPORT_FIND_LOAD_START_ENABLED(void)

int PyDTrace_IMPORT_FIND_LOAD_DONE_ENABLED(void)

int PyDTrace_AUDIT_ENABLED(void)

  All calls to `PyDTrace` functions must be guarded by a call to one of these functions. This allows Python to minimize performance impact when probing is disabled.

  On builds without DTrace enabled, these functions do nothing and return `0`.

## SystemTap Tapset

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.

## 使用例

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;
    }