linux-kernel - Re: [PATCH,RFC] perf: panic due to inclied cpu context task

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Date:	Mon, 28 Mar 2011 18:27:43 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx
 value

On Mon, 2011-03-28 at 17:15 +0200, Oleg Nesterov wrote:
> On 03/28, Peter Zijlstra wrote:
> >
> > --- linux-2.6.orig/kernel/perf_event.c
> > +++ linux-2.6/kernel/perf_event.c
> > @@ -1767,7 +1767,6 @@ static void ctx_sched_out(struct perf_ev
> >  	struct perf_event *event;
> >
> >  	raw_spin_lock(&ctx->lock);
> > -	perf_pmu_disable(ctx->pmu);
> >  	ctx->is_active = 0;
> >  	if (likely(!ctx->nr_events))
> >  		goto out;
> > @@ -1777,6 +1776,7 @@ static void ctx_sched_out(struct perf_ev
> >  	if (!ctx->nr_active)
> >  		goto out;
> >
> > +	perf_pmu_disable(ctx->pmu);
> >  	if (event_type & EVENT_PINNED) {
> >  		list_for_each_entry(event, &ctx->pinned_groups, group_entry)
> >  			group_sched_out(event, cpuctx, ctx);
> > @@ -1786,8 +1786,8 @@ static void ctx_sched_out(struct perf_ev
> >  		list_for_each_entry(event, &ctx->flexible_groups, group_entry)
> >  			group_sched_out(event, cpuctx, ctx);
> >  	}
> > -out:
> >  	perf_pmu_enable(ctx->pmu);
> > +out:
> >  	raw_spin_unlock(&ctx->lock);
> 
> Yes, thanks.
> 
> Probably this doesn't matter from the perfomance pov, but imho this
> makes the code more understandable. This is important for occasional
> readers like me ;)

Could actually save quite a lot of cycles, pmu-disable/enable can be
very expensive on some hardware.

> Could you answer another question? It is not immediately clear why
> ctx_sched_in() does not check nr_active != 0 before doing
> ctx_XXX_sched_in(). I guess, the only reason is perf_rotate_context()
> and the similar logic in perf_event_context_sched_in(). If we are
> doing, say, cpu_ctx_sched_out(FLEXIBLE) + cpu_ctx_sched_in(FLEXIBLE)
> then ->nr_active can be zero after cpu_ctx_sched_out().
> 
> Is my understanding correct? Or is there another reason?

nr_active counts the number of events that have been scheduled in, so
its perfectly fine to have either nr_active or !nr_active at that
point. 
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