linux-kernel - Re: [PATCH] perf_events: fix read() bogus counts when in error state

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Date:	Thu, 26 Nov 2009 18:36:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	perfmon2-devel@...ts.sourceforge.net, eranian@...il.com
Subject: Re: [PATCH] perf_events: fix read() bogus counts when in error
 state

On Thu, 2009-11-26 at 09:24 -0800, Stephane Eranian wrote:
> 	When a pinned group cannot be scheduled it goes into error state.
> 	Normally a group cannot go out of error state without being explicitly
> 	re-enabled or disabled. There was a bug in per-thread mode, whereby
> 	upon termination of the thread, the group would transition from error
> 	to off leading to bogus counts and timing information returned by
> 	read().

> 	Signed-off-by: Stephane Eranian <eranian@...gle.com>

Right, good catch, totally forgot about error state :/

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
	
> ---
>  perf_event.c |   11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 0b0d5f7..7a8bb5b 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -333,7 +333,16 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
>  		event->group_leader->nr_siblings--;
>  
>  	update_event_times(event);
> -	event->state = PERF_EVENT_STATE_OFF;
> +
> +	/*
> +	 * If event was in error state, then keep it
> +	 * that way, otherwise bogus counts will be
> +	 * returned on read(). The only way to get out
> +	 * of error state is by explicit re-enabling
> +	 * of the event
> +	 */
> +	if (event->state > PERF_EVENT_STATE_OFF)
> +		event->state = PERF_EVENT_STATE_OFF;
>  
>  	/*
>  	 * If this was a group event with sibling events then


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