Re: [ltt-dev] trace a futex

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

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* Jan Kiszka (jan.kiszka@web.de) wrote:
> Mathieu Desnoyers wrote:
> > * Gian Lorenzo Meocci (glmeocci@gmail.com) wrote:
> >> Hi Mathieu,
> >>
> >> thanks for your reply.
> >>
> >> I want specify that:
> >> 1) I am already patching glibc (and, of course, nptl/pthread_*)
> >> 2) I already add two event to pthread_mutex_lock. The first at the
> >> beginning of the function and the second after all return 0 presented
> >> on that function. But those two events are not enough to establish if
> >> a pthread_mutex_lock has been blocking.
> >> In fact I know only the time spent on pthread_mutex_lock. If this time
> >> is little probably I hold the mutex otherwise I was been descheduled.
> >>
> >> So thanks a lot again,
> >>
> >>
> > 
> > Ok, then you will probably want to correlate your information with :
> > 
> > - scheduling activity regarding your threads
> > - system call entry events, especially sys_futex. Note that a thread
> >   calling sys_futex won't _necessarily_ be put to sleep.. it may still
> >   be able to take the lock relatively quickly.
> > 
> > If you need more than that, we may think of instrumenting futex.c, but I
> > am not sure this is required.
> 
> Haven't checked the state of instrumentation recently: Is the futex
> operation visible in the trace now? It used to be not, and we often had
> to guess the reason for sys_futex (wake, wait, pi or not pi, etc.) from
> the context - or add ad-hoc instrumentation.
> 
> Jan
> 

It still isn't instrumented, and I think it would be good to add such
instrumentation.

Have a look at the patch done by K. Prasad for futex.c : it should
probably be updated so it uses tracepoints instead of markers. Anyone
would like to do this ?

See http://lkml.org/lkml/2008/4/15/125
"[RFC PATCH 0/2] Debugging infrastructure for Futexes using Markers"

Mathieu

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Mathieu Desnoyers
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