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

[Song Liu]

> On Mar 29, 2021, at 4:33 AM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> 
> On Mon, Mar 29, 2021 at 2:17 AM Song Liu <songliubraving@xxxxxx> wrote:
>>> On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhyung@xxxxxxxxxx> 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). 

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?

Thanks,
Song