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Subject: Re: [PATCH] perf_event: fix slow and broken cgroup context switch
 code
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, ming.m.lin@intel.com,
        Oleg Nesterov <oleg@redhat.com>, Jiri Olsa <jolsa@redhat.com>
Date: Thu, 25 Aug 2011 16:42:01 +0200
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On Thu, 2011-08-25 at 16:36 +0200, Stephane Eranian wrote:
> On Thu, Aug 25, 2011 at 4:20 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, 2011-08-25 at 15:58 +0200, Stephane Eranian wrote:
> >> +static inline void perf_event_task_sched_out(struct task_struct
> >> *prev,
> >> +                                            struct task_struct *next)
> >>  {
> >>         perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, NULL, 0);
> >>
> >> -       __perf_event_task_sched_out(task, next);
> >> +       if (static_branch(&perf_sched_events))
> >> +               __perf_event_task_sched_out(prev, next);
> >>  }
> >
> > Right, so the reason we removed the static branch from there is
> >
> >  lkml.kernel.org/r/20110324164436.GC1930@jolsa.brq.redhat.com
> >
> > now I think the series 075e0b0085 to 64ce312618e should have cured that
> > problem, and adding the static_branch() is now safe again. But there's
> > no mention of any of this in the Changelog.
> >
> I realized I did not talk about the static_branch() change after I had
> clicked on
> Send. But to me, this looks natural to have the static branch in the ctxsw out
> routine. This has to be symmetrical with ctxsw in . The static branch is about
> avoiding perf ctxsw when there is no need for it, i.e., no per-thread
> nor per-cgroup
> events.

Yeah, that argument is what got us into trouble initially :) But I think
its ok now, we'll see if stuff explodes or not..