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  <div class="section" id="instrumenting-cpython-with-dtrace-and-systemtap">
<span id="instrumentation"></span><h1>Instrumenting CPython with DTrace and SystemTap<a class="headerlink" href="#instrumenting-cpython-with-dtrace-and-systemtap" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">author:</th><td class="field-body">David Malcolm</td>
</tr>
<tr class="field-even field"><th class="field-name">author:</th><td class="field-body">Łukasz Langa</td>
</tr>
</tbody>
</table>
<p>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:</p>
<blockquote>
<div><ul class="simple">
<li>filter which processes are to be observed</li>
<li>gather data from the processes of interest</li>
<li>generate reports on the data</li>
</ul>
</div></blockquote>
<p>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.</p>
<div class="impl-detail compound">
<p><strong>CPython implementation detail:</strong> 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.</p>
</div>
<div class="section" id="enabling-the-static-markers">
<h2>Enabling the static markers<a class="headerlink" href="#enabling-the-static-markers" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>On a Linux machine, this can be done via:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> yum install systemtap-sdt-devel
</pre></div>
</div>
<p>or:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> sudo apt-get install systemtap-sdt-dev
</pre></div>
</div>
<p>CPython must then be configured <code class="docutils literal notranslate"><span class="pre">--with-dtrace</span></code>:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>checking for --with-dtrace... yes
</pre></div>
</div>
<p>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:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3.6 -q <span class="p">&amp;</span>
<span class="gp">$</span> sudo dtrace -l -P python<span class="nv">$!</span>  <span class="c1"># or: dtrace -l -m python3.6</span>

<span class="go">   ID   PROVIDER            MODULE                          FUNCTION NAME</span>
<span class="go">29564 python18035        python3.6          _PyEval_EvalFrameDefault function-entry</span>
<span class="go">29565 python18035        python3.6             dtrace_function_entry function-entry</span>
<span class="go">29566 python18035        python3.6          _PyEval_EvalFrameDefault function-return</span>
<span class="go">29567 python18035        python3.6            dtrace_function_return function-return</span>
<span class="go">29568 python18035        python3.6                           collect gc-done</span>
<span class="go">29569 python18035        python3.6                           collect gc-start</span>
<span class="go">29570 python18035        python3.6          _PyEval_EvalFrameDefault line</span>
<span class="go">29571 python18035        python3.6                 maybe_dtrace_line line</span>
</pre></div>
</div>
<p>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.</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> readelf -S ./python <span class="p">|</span> grep .note.stapsdt
<span class="go">[30] .note.stapsdt        NOTE         0000000000000000 00308d78</span>
</pre></div>
</div>
<p>If you’ve built Python as a shared library (with –enable-shared), you
need to look instead within the shared library.  For example:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> readelf -S libpython3.3dm.so.1.0 <span class="p">|</span> grep .note.stapsdt
<span class="go">[29] .note.stapsdt        NOTE         0000000000000000 00365b68</span>
</pre></div>
</div>
<p>Sufficiently modern readelf can print the metadata:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> readelf -n ./python

<span class="go">Displaying notes found at file offset 0x00000254 with length 0x00000020:</span>
<span class="go">    Owner                 Data size          Description</span>
<span class="go">    GNU                  0x00000010          NT_GNU_ABI_TAG (ABI version tag)</span>
<span class="go">        OS: Linux, ABI: 2.6.32</span>

<span class="go">Displaying notes found at file offset 0x00000274 with length 0x00000024:</span>
<span class="go">    Owner                 Data size          Description</span>
<span class="go">    GNU                  0x00000014          NT_GNU_BUILD_ID (unique build ID bitstring)</span>
<span class="go">        Build ID: df924a2b08a7e89f6e11251d4602022977af2670</span>

<span class="go">Displaying notes found at file offset 0x002d6c30 with length 0x00000144:</span>
<span class="go">    Owner                 Data size          Description</span>
<span class="go">    stapsdt              0x00000031          NT_STAPSDT (SystemTap probe descriptors)</span>
<span class="go">        Provider: python</span>
<span class="go">        Name: gc__start</span>
<span class="go">        Location: 0x00000000004371c3, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf6</span>
<span class="go">        Arguments: -4@%ebx</span>
<span class="go">    stapsdt              0x00000030          NT_STAPSDT (SystemTap probe descriptors)</span>
<span class="go">        Provider: python</span>
<span class="go">        Name: gc__done</span>
<span class="go">        Location: 0x00000000004374e1, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bf8</span>
<span class="go">        Arguments: -8@%rax</span>
<span class="go">    stapsdt              0x00000045          NT_STAPSDT (SystemTap probe descriptors)</span>
<span class="go">        Provider: python</span>
<span class="go">        Name: function__entry</span>
<span class="go">        Location: 0x000000000053db6c, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6be8</span>
<span class="go">        Arguments: 8@%rbp 8@%r12 -4@%eax</span>
<span class="go">    stapsdt              0x00000046          NT_STAPSDT (SystemTap probe descriptors)</span>
<span class="go">        Provider: python</span>
<span class="go">        Name: function__return</span>
<span class="go">        Location: 0x000000000053dba8, Base: 0x0000000000630ce2, Semaphore: 0x00000000008d6bea</span>
<span class="go">        Arguments: 8@%rbp 8@%r12 -4@%eax</span>
</pre></div>
</div>
<p>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.</p>
</div>
<div class="section" id="static-dtrace-probes">
<h2>Static DTrace probes<a class="headerlink" href="#static-dtrace-probes" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>self int indent;

python$target:::function-entry
/copyinstr(arg1) == &quot;start&quot;/
{
        self-&gt;trace = 1;
}

python$target:::function-entry
/self-&gt;trace/
{
        printf(&quot;%d\t%*s:&quot;, timestamp, 15, probename);
        printf(&quot;%*s&quot;, self-&gt;indent, &quot;&quot;);
        printf(&quot;%s:%s:%d\n&quot;, basename(copyinstr(arg0)), copyinstr(arg1), arg2);
        self-&gt;indent++;
}

python$target:::function-return
/self-&gt;trace/
{
        self-&gt;indent--;
        printf(&quot;%d\t%*s:&quot;, timestamp, 15, probename);
        printf(&quot;%*s&quot;, self-&gt;indent, &quot;&quot;);
        printf(&quot;%s:%s:%d\n&quot;, basename(copyinstr(arg0)), copyinstr(arg1), arg2);
}

python$target:::function-return
/copyinstr(arg1) == &quot;start&quot;/
{
        self-&gt;trace = 0;
}
</pre></div>
</div>
<p>It can be invoked like this:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> sudo dtrace -q -s call_stack.d -c <span class="s2">&quot;python3.6 script.py&quot;</span>
</pre></div>
</div>
<p>The output looks like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>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
</pre></div>
</div>
</div>
<div class="section" id="static-systemtap-markers">
<h2>Static SystemTap markers<a class="headerlink" href="#static-systemtap-markers" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>For example, this SystemTap script can be used to show the call/return
hierarchy of a Python script:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>probe process(&quot;python&quot;).mark(&quot;function__entry&quot;) {
     filename = user_string($arg1);
     funcname = user_string($arg2);
     lineno = $arg3;

printf(&quot;%s =&gt; %s in %s:%d\\n&quot;,
            thread_indent(1), funcname, filename, lineno);
}

probe process(&quot;python&quot;).mark(&quot;function__return&quot;) {
    filename = user_string($arg1);
    funcname = user_string($arg2);
    lineno = $arg3;

printf(&quot;%s &lt;= %s in %s:%d\\n&quot;,
           thread_indent(-1), funcname, filename, lineno);
}
</pre></div>
</div>
<p>It can be invoked like this:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> stap <span class="se">\</span>
  show-call-hierarchy.stp <span class="se">\</span>
  -c <span class="s2">&quot;./python test.py&quot;</span>
</pre></div>
</div>
<p>The output looks like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>11408 python(8274):        =&gt; __contains__ in Lib/_abcoll.py:362
11414 python(8274):         =&gt; __getitem__ in Lib/os.py:425
11418 python(8274):          =&gt; encode in Lib/os.py:490
11424 python(8274):          &lt;= encode in Lib/os.py:493
11428 python(8274):         &lt;= __getitem__ in Lib/os.py:426
11433 python(8274):        &lt;= __contains__ in Lib/_abcoll.py:366
</pre></div>
</div>
<p>where the columns are:</p>
<blockquote>
<div><ul class="simple">
<li>time in microseconds since start of script</li>
<li>name of executable</li>
<li>PID of process</li>
</ul>
</div></blockquote>
<p>and the remainder indicates the call/return hierarchy as the script executes.</p>
<p>For a <cite>–enable-shared</cite> 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:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>probe process(&quot;python&quot;).mark(&quot;function__entry&quot;) {
</pre></div>
</div>
<p>should instead read:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>probe process(&quot;python&quot;).library(&quot;libpython3.6dm.so.1.0&quot;).mark(&quot;function__entry&quot;) {
</pre></div>
</div>
<p>(assuming a debug build of CPython 3.6)</p>
</div>
<div class="section" id="available-static-markers">
<h2>Available static markers<a class="headerlink" href="#available-static-markers" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="c.function__entry">
<code class="descname">function__entry</code><span class="sig-paren">(</span>str<em>&nbsp;filename</em>, str<em>&nbsp;funcname</em>, int<em>&nbsp;lineno</em><span class="sig-paren">)</span><a class="headerlink" href="#c.function__entry" title="Permalink to this definition">¶</a></dt>
<dd><p>This marker indicates that execution of a Python function has begun.
It is only triggered for pure-Python (bytecode) functions.</p>
<p>The filename, function name, and line number are provided back to the
tracing script as positional arguments, which must be accessed using
<code class="docutils literal notranslate"><span class="pre">$arg1</span></code>, <code class="docutils literal notranslate"><span class="pre">$arg2</span></code>, <code class="docutils literal notranslate"><span class="pre">$arg3</span></code>:</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">$arg1</span></code> : <code class="docutils literal notranslate"><span class="pre">(const</span> <span class="pre">char</span> <span class="pre">*)</span></code> filename, accessible using <code class="docutils literal notranslate"><span class="pre">user_string($arg1)</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">$arg2</span></code> : <code class="docutils literal notranslate"><span class="pre">(const</span> <span class="pre">char</span> <span class="pre">*)</span></code> function name, accessible using
<code class="docutils literal notranslate"><span class="pre">user_string($arg2)</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">$arg3</span></code> : <code class="docutils literal notranslate"><span class="pre">int</span></code> line number</li>
</ul>
</div></blockquote>
</dd></dl>

<dl class="function">
<dt id="c.function__return">
<code class="descname">function__return</code><span class="sig-paren">(</span>str<em>&nbsp;filename</em>, str<em>&nbsp;funcname</em>, int<em>&nbsp;lineno</em><span class="sig-paren">)</span><a class="headerlink" href="#c.function__return" title="Permalink to this definition">¶</a></dt>
<dd><p>This marker is the converse of <a class="reference internal" href="#c.function__entry" title="function__entry"><code class="xref c c-func docutils literal notranslate"><span class="pre">function__entry()</span></code></a>, and indicates that
execution of a Python function has ended (either via <code class="docutils literal notranslate"><span class="pre">return</span></code>, or via an
exception).  It is only triggered for pure-Python (bytecode) functions.</p>
<p>The arguments are the same as for <a class="reference internal" href="#c.function__entry" title="function__entry"><code class="xref c c-func docutils literal notranslate"><span class="pre">function__entry()</span></code></a></p>
</dd></dl>

<dl class="function">
<dt id="c.line">
<code class="descname">line</code><span class="sig-paren">(</span>str<em>&nbsp;filename</em>, str<em>&nbsp;funcname</em>, int<em>&nbsp;lineno</em><span class="sig-paren">)</span><a class="headerlink" href="#c.line" title="Permalink to this definition">¶</a></dt>
<dd><p>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.</p>
<p>The arguments are the same as for <a class="reference internal" href="#c.function__entry" title="function__entry"><code class="xref c c-func docutils literal notranslate"><span class="pre">function__entry()</span></code></a>.</p>
</dd></dl>

<dl class="function">
<dt id="c.gc__start">
<code class="descname">gc__start</code><span class="sig-paren">(</span>int<em>&nbsp;generation</em><span class="sig-paren">)</span><a class="headerlink" href="#c.gc__start" title="Permalink to this definition">¶</a></dt>
<dd><p>Fires when the Python interpreter starts a garbage collection cycle.
<code class="docutils literal notranslate"><span class="pre">arg0</span></code> is the generation to scan, like <a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate"><span class="pre">gc.collect()</span></code></a>.</p>
</dd></dl>

<dl class="function">
<dt id="c.gc__done">
<code class="descname">gc__done</code><span class="sig-paren">(</span>long<em>&nbsp;collected</em><span class="sig-paren">)</span><a class="headerlink" href="#c.gc__done" title="Permalink to this definition">¶</a></dt>
<dd><p>Fires when the Python interpreter finishes a garbage collection
cycle. <code class="docutils literal notranslate"><span class="pre">arg0</span></code> is the number of collected objects.</p>
</dd></dl>

</div>
<div class="section" id="systemtap-tapsets">
<h2>SystemTap Tapsets<a class="headerlink" href="#systemtap-tapsets" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>Here is a tapset file, based on a non-shared build of CPython:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>/*
   Provide a higher-level wrapping around the function__entry and
   function__return markers:
 \*/
probe python.function.entry = process(&quot;python&quot;).mark(&quot;function__entry&quot;)
{
    filename = user_string($arg1);
    funcname = user_string($arg2);
    lineno = $arg3;
    frameptr = $arg4
}
probe python.function.return = process(&quot;python&quot;).mark(&quot;function__return&quot;)
{
    filename = user_string($arg1);
    funcname = user_string($arg2);
    lineno = $arg3;
    frameptr = $arg4
}
</pre></div>
</div>
<p>If this file is installed in SystemTap’s tapset directory (e.g.
<code class="docutils literal notranslate"><span class="pre">/usr/share/systemtap/tapset</span></code>), then these additional probepoints become
available:</p>
<dl class="function">
<dt id="c.python.function.entry">
<code class="descname">python.function.entry</code><span class="sig-paren">(</span>str<em>&nbsp;filename</em>, str<em>&nbsp;funcname</em>, int<em>&nbsp;lineno</em>, frameptr<span class="sig-paren">)</span><a class="headerlink" href="#c.python.function.entry" title="Permalink to this definition">¶</a></dt>
<dd><p>This probe point indicates that execution of a Python function has begun.
It is only triggered for pure-python (bytecode) functions.</p>
</dd></dl>

<dl class="function">
<dt id="c.python.function.return">
<code class="descname">python.function.return</code><span class="sig-paren">(</span>str<em>&nbsp;filename</em>, str<em>&nbsp;funcname</em>, int<em>&nbsp;lineno</em>, frameptr<span class="sig-paren">)</span><a class="headerlink" href="#c.python.function.return" title="Permalink to this definition">¶</a></dt>
<dd><p>This probe point is the converse of <a class="reference internal" href="#c.python.function.return" title="python.function.return"><code class="xref c c-func docutils literal notranslate"><span class="pre">python.function.return()</span></code></a>, and
indicates that execution of a Python function has ended (either via
<code class="docutils literal notranslate"><span class="pre">return</span></code>, or via an exception).  It is only triggered for pure-python
(bytecode) functions.</p>
</dd></dl>

</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>probe python.function.entry
{
  printf(&quot;%s =&gt; %s in %s:%d\n&quot;,
         thread_indent(1), funcname, filename, lineno);
}

probe python.function.return
{
  printf(&quot;%s &lt;= %s in %s:%d\n&quot;,
         thread_indent(-1), funcname, filename, lineno);
}
</pre></div>
</div>
<p>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:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>global fn_calls;

probe python.function.entry
{
    fn_calls[pid(), filename, funcname, lineno] += 1;
}

probe timer.ms(1000) {
    printf(&quot;\033[2J\033[1;1H&quot;) /* clear screen \*/
    printf(&quot;%6s %80s %6s %30s %6s\n&quot;,
           &quot;PID&quot;, &quot;FILENAME&quot;, &quot;LINE&quot;, &quot;FUNCTION&quot;, &quot;CALLS&quot;)
    foreach ([pid, filename, funcname, lineno] in fn_calls- limit 20) {
        printf(&quot;%6d %80s %6d %30s %6d\n&quot;,
            pid, filename, lineno, funcname,
            fn_calls[pid, filename, funcname, lineno]);
    }
    delete fn_calls;
}
</pre></div>
</div>
</div>
</div>

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  <h3><a href="../contents.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">Instrumenting CPython with DTrace and SystemTap</a><ul>
<li><a class="reference internal" href="#enabling-the-static-markers">Enabling the static markers</a></li>
<li><a class="reference internal" href="#static-dtrace-probes">Static DTrace probes</a></li>
<li><a class="reference internal" href="#static-systemtap-markers">Static SystemTap markers</a></li>
<li><a class="reference internal" href="#available-static-markers">Available static markers</a></li>
<li><a class="reference internal" href="#systemtap-tapsets">SystemTap Tapsets</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
</ul>
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