linux-kernel - Re: [PATCH] perf: do not set task_ctx pointer in cpuctx if there is no events in the context

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Message-ID: <20111103074104.GG14726@redhat.com>
Date:	Thu, 3 Nov 2011 09:41:04 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH] perf: do not set task_ctx pointer in cpuctx if there is
 no events in the context

Ingo, Peter can you look into this please.

On Sun, Oct 23, 2011 at 07:10:33PM +0200, Gleb Natapov wrote:
> Do not set task_ctx pointer during sched_in if there is no
> events associated with the context.  Otherwise if during task
> execution total number of events in the system will become zero
> perf_event_context_sched_out() will not be called and cpuctx->task_ctx
> will be left with a stale value.
> 
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 0f85778..68b13ee 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2172,7 +2172,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>  
>  	perf_event_sched_in(cpuctx, ctx, task);
>  
> -	cpuctx->task_ctx = ctx;
> +	if (ctx->nr_events)
> +		cpuctx->task_ctx = ctx;
>  
>  	perf_pmu_enable(ctx->pmu);
>  	perf_ctx_unlock(cpuctx, ctx);
> --
> 			Gleb.
> --
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--
			Gleb.
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