linux-kernel - Re: WARNING: at arch/x86/kernel/cpu/perf

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Date:	Mon, 6 Feb 2012 15:32:46 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>
Subject: Re: WARNING: at arch/x86/kernel/cpu/perf_event.c:989

On Mon, Feb 6, 2012 at 3:29 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 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!
>
I am wondering why we stop and restart the code in perf_adjust_period()
when it's called from __perf_event_overflow(). Isn't it supposed to be stopped
already by the model specific interrupt handler. Looks like we do stop/start,
just to get the reload aspect of start. Is that right?
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