linux-kernel - Re: [PATCH 3/4] perf, nmi: Move LVT un-masking into irq handlers

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Date:	Mon, 25 Apr 2011 18:51:54 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	George Spelvin <linux@...izon.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/4] perf, nmi: Move LVT un-masking into irq handlers

On 04/25/2011 06:50 PM, Don Zickus wrote:
> On Mon, Apr 25, 2011 at 06:15:25PM +0400, Cyrill Gorcunov wrote:
>>
>>   Don't get me wrong please but the whole picture of what is happening can be seen only when
>> all caller sequence is taken into account and once (for some reason) the sequence
>> get changed the "detailed" comment would simply mess the comment reader so I think
>> the former comment is detailed enough and what is more important it's "general" enough
>> so it doesn't depend on when code is called but points a reader on hw details and it's
>> up to a reader to check "current" calling sequence because kernel code changes too
>> damn fast ;)
> 
> I think Ingo is looking for is something like
> 
> /*
>  * It has been observed that quirks in the P4 perf hw has forced the
>  * following sequence of events to happen in the order below
>  *
>  * - clear the OVF bit (as it will continue to assert the NMI line)
>  * - unmask the apic LVTPC bit to allow NMIs from the PMU again
>  * - optionally re-enable the PMU to count events again
>  *
>  * Un-masking the apic prematurely (before clearing the OVF bit) has led
>  * to the creation of a second NMI event (which led to the unknown NMI
>  * warnings) due to the fact that the PMU will continue to generate an
>  * interrupt until its OVF bit is cleared.
>  */
> 
> Something that specifically documents what we saw, why we saw it and what
> we are doing to avoid it.
> 
> Cheers,
> Don

Excellent, thanks a huge Don!

-- 
    Cyrill
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