[lttng-dev] LTTng user-space callstacks

[Valentin Grigorev]

Hmm, I don't know unfortunately, it should be checked

Best regards,
Valentin Grigorev


On Fri, Mar 27, 2020 at 4:55 PM Mathieu Desnoyers <
mathieu.desnoyers at efficios.com> wrote:

> Last time I checked, the main issue with perf's handling of cases that
> lack frame pointers
> is that they grab a ~2-4kB worth of stack data into the trace, and then
> post-process it
> with libunwind. This might be OK for sampling use-cases, but is not
> something I consider
> appropriate for tracing due to considerably increased use of tracing
> throughput.
>
> Has the situation changed on the perf side ?
>
> Thanks,
>
> Mathieu
>
> ----- On Mar 27, 2020, at 9:44 AM, Valentin Grigorev <
> valentin.grigorev at jetbrains.com> wrote:
>
> Yeah, as far as I know, perf [1] is successfully using libunwind to
> provide callstacks. Probably, LTTng team should think about it too, if
> there are no serious obstacles.
>
> [1]: https://perf.wiki.kernel.org/index.php/Main_Page
>
>
> Best regards,
> Valentin Grigorev
>
>
> On Thu, Mar 26, 2020 at 11:53 PM Milian Wolff <milian.wolff at kdab.com>
> wrote:
>
>> On Donnerstag, 26. März 2020 20:41:17 CET Mathieu Desnoyers via lttng-dev
>> wrote:
>> > ----- On Mar 26, 2020, at 1:39 PM, lttng-dev <lttng-dev at lists.lttng.org>
>>
>> wrote:
>> > > Hello!
>> > >
>> > > Currently, callstack collection in LTTng is only available for
>> > > kernel-space
>> > > events with context fields callstack-kernel and callstack-user .
>> > >
>> > > Is it expected that callstack collection for LTTng-UST will be added
>> too?
>> > > And if it is expected, then how soon?
>> >
>> > Hi Valentin,
>> >
>> > It is something that would be interesting and useful, but a lot of work
>> > would be needed to have stack-walking this is fast enough and reentrant
>> wrt
>> > signal handlers. Unfortunately, the backtrace(3) functions do not meet
>> > those requirements.
>> >
>> > Also, for gathering user-space callstacks from the kernel tracer, it
>> only
>> > works if all user-space is compiled with frame pointers. This is also a
>> > limitation that would require a lot of work to overcome neatly.
>> >
>> > None of those features are currently on any roadmap due to lack of
>> customers
>> > showing interest in getting this done.
>>
>> libunwind [1] should meet the requirements you need. It is high
>> performance
>> thanks to extensive caching, signal safe and does not rely on frame
>> pointers.
>>
>> [1]: https://github.com/libunwind/libunwind
>>
>> --
>> Milian Wolff | milian.wolff at kdab.com | Senior Software Engineer
>> KDAB (Deutschland) GmbH, a KDAB Group company
>> Tel: +49-30-521325470
>> KDAB - The Qt, C++ and OpenGL Experts
>
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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