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Subject: Re: WARNING: at arch/x86/kernel/cpu/perf_event.c:989
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Stephane Eranian <eranian@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Mon, 06 Feb 2012 15:29:28 +0100
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On Mon, 2012-02-06 at 15:13 +0100, Stephane Eranian wrote:
> Ok, I found the problem!
> 
> it comes from perf_adjust_freq_unthr_context() vs perf_adjust_period().
> The latter can under certain condition stop and restart the event. So we
> had:
> 
>       stop()
>       if (delta > 0) {
>            perf_adjust_period() {
>                if (period > 8*...) {
>                    stop()
>                    ...
>                    start()
>                }
>            }
>        }
>        start()
> 
> Could have a double stop() and double start(), thus triggering the warning in
> x86_pmu_start().
> 
> Will post a patch shortly to fix this. 

Nice, thanks for looking at this!