[ltt-dev] trace a futex

[Jan Kiszka]

Mathieu Desnoyers wrote:
> * Gian Lorenzo Meocci (glmeocci at 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

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