Re: [RFC PATCH 0/2] perf_events: add support for per-cpu per-cgroup monitoring (v3)

Stephane Eranian
2010-09-21T16:33:42Z

On Tue, Sep 21, 2010 at 6:27 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, 2010-09-21 at 18:17 +0200, Stephane Eranian wrote:
>> On Tue, Sep 21, 2010 at 4:03 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > On Tue, 2010-09-21 at 15:38 +0200, Stephane Eranian wrote:
>> >> > Hmm, indeed. One thing we can do about that is move perf into the
>> >> > cgroup, create the counter (disabled) using self to identify the cgroup,
>> >> > move perf back to where it came from, and enable the counter.
>> >> >
>> >> Yes, that's another possibility. I wonder if there are any non-obvious
>> >> difficulties with this approach.
>> >
>> > Yes, there is, but I think we can fix it. The problem with moving perf
>> > itself around is that perf is not a fully dormant process and can thus
>> > interact with the cgroup state.
>> >
>> I was thinking about memory accounting for instance.
>
> I think the memory controller only accounts things when the process
> actually touches something. A process that never wakes will never touch
> anything.
>
>> > If we were to fork a child that's simply sitting idle in waitpid() (or
>> > any other blocking syscall) we can move that around cgroup without
>> > affecting the cgroup itself.
>>
>> But then things get a bit more complicated because the perf_event_open()
>> has to be done in that child. File descriptors created in child processes
>> and not shared with their parent. You'd have to pass file descriptors around.
>> That seems overly complicated.
>
> Uhm, no the trick is that the child remains absolutely dormant and
> therefore doesn't accrue any accounting, all you need is a known task in
> the cgroup, the parent can then specify the child pid to identify the
> group.
>
> Once you've opened the counter, you can move the kid out and kill it.
> Note that moving it out of the cgroup before killing it ensure it never
> wakes up inside that cgroup.
>
>
Ok, I see. I got confused by the 'self' pid. The parent moves the dormant
child into the cgroup and uses its pid in perf_event_open(). The existing
logic inside the tool remains unmodified.

I will experiment with this approach.