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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks Francis.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">This is what I expected to have to do. I do agree though that adding this to lttng-ust would be a good way to go.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Should we end up on this path, it certainly seems like it might we worth our time to investigate what it would take to add it to lttng-ust. Do you know who is
the right person to talk to about this? I’d want to make sure that this would not be a non-starter.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Brian
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Francis Giraldeau [mailto:francis.giraldeau@gmail.com]
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<b>Sent:</b> Monday, March 9, 2015 12:39 PM<br>
<b>To:</b> Brian Robbins<br>
<b>Cc:</b> lttng-dev@lists.lttng.org<br>
<b>Subject:</b> Re: [lttng-dev] Userspace Tracing and Backtraces<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">2015-03-06 13:51 GMT-05:00 Brian Robbins <<a href="mailto:brianrob@microsoft.com" target="_blank">brianrob@microsoft.com</a>>:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks Francis.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Is it accurate to say then that the array of addresses would need to be captured by app code by writing
a stack walker by hand</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Yes, the callstack can be recorded in userspace. You would need a tracepoint with a varying length field:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">TRACEPOINT_EVENT(myprovider, callstack,<o:p></o:p></p>
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<p class="MsoNormal"> TP_ARGS(unsigned long *, buf, size_t, depth),<o:p></o:p></p>
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<p class="MsoNormal"> TP_FIELDS(<o:p></o:p></p>
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<p class="MsoNormal"> ctf_sequence(unsigned long, addr, buf, size_t, depth)<o:p></o:p></p>
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<p class="MsoNormal"> )<o:p></o:p></p>
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<p class="MsoNormal">)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In the app code, use libunwind [1] to get the addresses, then call the tracepoint: <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> do_unwind() // use libunwind here<o:p></o:p></p>
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<p class="MsoNormal"> tracepoint(myprovider, buf, depth);<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">However, the unwind will be done whether or not the tracepoint is active (~10us-100us in steady state, so it's may become expansive if called often). I know there was discussion about tp_code() for such use case (some code to call before
the tracepoint only if it is enabled). Or you can cheat: <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">if (__builtin_expect(!!(__tracepoint_myprovider___callstack.state), 0)) {<o:p></o:p></p>
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<p class="MsoNormal"> do_unwind(...)<o:p></o:p></p>
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<p class="MsoNormal"> tracepoint(myprovider, buf, depth);<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">That said, instead of having a callstack tracepoint, IMHO the best solution would be instead extending lttng-ust to add callstack event context (itself linked to libunwind). Then, recording the callstack would be simple like that:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> $ lttng add-context -u -t callstack<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">or using the perf capture mechanism that you describe below?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Perf is peeking at the userspace from kernel space, it's another story. I guess that libunwind was not ported to the kernel because it is a large chunk of complicated code that performs a lot of I/O and computation, while copying a portion
of the stack is really about KISS and low runtime overhead.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Francis<o:p></o:p></p>
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