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

[David Ahern <dsahern@gmail.com>]

On 7/22/13 12:50 AM, Manuel Selva wrote:
> 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 ?

Arnaldo is the current one I believe.
David


>
> 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
>>
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