Re: How does perf collects per thread/process events ?

[Manuel Selva <selva.manuel@gmail.com>]

Thanks for the answer Michael.

I just created an account to be able to edit the perf wiki page. Before 
doing that, I am asking here if someone knows the policy to update this 
wiki or who are the maintainers to ask them ?

Thanks,

Manu

On 07/22/2013 06:19 AM, Michael Ellerman wrote:
> On Tue, 2013-07-16 at 17:29 +0200, Manuel Selva wrote:
>> Hi,
>>
>> My question regards a platform equipped with 2 Intel Xeon X5650.
>> According to the perf wiki page
>> (https://perf.wiki.kernel.org/index.php/Tutorial), "by default perf stat
>> counts for all threads of the process and subsequent child processes and
>> threads" and "By default, perf stat counts in per-thread mode".
>>
>> So a first question is what is the default: per thread or per process ?
>
> It's per process, which is as described in the first quote above. The
> second quote is just wrong AFAICS.
>
>> Then, independently of the answer, I am wondering how does perf handles
>> per thread or per process regarding the scheduler and migrations. I
>> didn't find it explicitly in the Intel documentation but it seems
>> natural that hardware performance counters located on a given core are
>> only capable of counting event on this core and not on other cores. Is
>> it true ?
>>
>> Moreover, the wiki page says that "When a thread migrated from one
>> processor to another, counters are saved on the current processor and
>> are restored on the new one" (this seems to confirm the answer to my
>> previous question above). It means that the scheduler is aware about
>> "perf" or that perf is able to register a hook into the scheduler. So I
>> guess this is done in the kernel part of perf (in the implementation of
>> the perf_event_open system call) and not in the user land part, is it true ?
>
> Yes.
>
> cheers
>