Re: [PATCH 1/2] perf/core: Share an event with multiple cgroups

[Song Liu <songliubraving@fb.com>]

> On Mar 30, 2021, at 8:11 AM, Namhyung Kim <namhyung@kernel.org> wrote:
> 
> On Tue, Mar 30, 2021 at 3:33 PM Song Liu <songliubraving@fb.com> wrote:
>>> On Mar 29, 2021, at 4:33 AM, Namhyung Kim <namhyung@kernel.org> wrote:
>>> 
>>> On Mon, Mar 29, 2021 at 2:17 AM Song Liu <songliubraving@fb.com> wrote:
>>>>> On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhyung@kernel.org> wrote:
>>>>> 
>>>>> As we can run many jobs (in container) on a big machine, we want to
>>>>> measure each job's performance during the run.  To do that, the
>>>>> perf_event can be associated to a cgroup to measure it only.
>>>>> 
>> 
>> [...]
>> 
>>>>> +     return 0;
>>>>> +}
>>>> 
>>>> Could you please explain why we need this logic in can_attach?
>>> 
>>> IIUC the ss->attach() is called after a task's cgroup membership
>>> is changed.  But we want to collect the performance numbers for
>>> the old cgroup just before the change.  As the logic merely checks
>>> the current task's cgroup, it should be done in the can_attach()
>>> which is called before the cgroup change.
>> 
>> Thanks for the explanations.
>> 
>> Overall, I really like the core idea, especially that the overhead on
>> context switch is bounded (by the depth of cgroup tree).
> 
> Thanks!
> 
>> 
>> Is it possible to make PERF_EVENT_IOC_ATTACH_CGROUP more flexible?
>> Specifically, if we can have
>> 
>>  PERF_EVENT_IOC_ADD_CGROUP     add a cgroup to the list
>>  PERF_EVENT_IOC_EL_CGROUP      delete a cgroup from the list
>> 
>> we can probably share these events among multiple processes, and
>> these processes don't need to know others' cgroup list. I think
>> this will be useful for users to build customized monitoring in
>> its own container.
>> 
>> Does this make sense?
> 
> Maybe we can add ADD/DEL interface for more flexible monitoring
> but I'm not sure which use cases it'll be used actually.
> 
> For your multi-process sharing case, the original events' file
> descriptors should be shared first.  

Yes, we will need some other work to make the ADD/DEL interface 
work properly. Let's worry about that later. 

For both patches:

Acked-by: Song Liu <songliubraving@fb.com>

Thanks,
Song

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