Re: [PATCH] perf_event: fix slow and broken cgroup context switch code

[Stephane Eranian <eranian@google.com>]

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.