linux-kernel - Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event on intel perf counter

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Message-ID: <AANLkTik+QC8YrOHRajUPP=eKph06yOmqLOEhx=2387HH@mail.gmail.com>
Date:	Fri, 3 Sep 2010 16:28:26 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event
 on intel perf counter

On Fri, Sep 3, 2010 at 4:03 PM, Don Zickus <dzickus@...hat.com> wrote:
> On Fri, Sep 03, 2010 at 01:02:49PM +0200, Stephane Eranian wrote:
>> Here is an example of what I gathered on a Westmere:
>>
>> This is coming into the interrupt handler:
>> - status   = overflow status coming from GLOBAL_OVF_STATUS
>> - status2 = inspection of the counters
>> - act = cpuc->active_mask[0]
>>
>> In case both status don't match, I dump the state of the active events
>> incl. the counter values(val).
>>
>> [  822.813808] CPU2 irqin status=0x6 status2=0x4 act=0x7
>> [  822.813818] CPU2 cfg=0x13003c idx=0 sel=53003c val=ffffa833f298
>> [  822.813821] CPU2 cfg=0x12003c idx=1 sel=52003c val=fffffe130229
>> [  822.813823] CPU2 cfg=0x11003c idx=2 sel=51003c val=5e9
>>
>> Here only counter2 has overflowed, yet the handler will also process counter1
>> which is wrong.
>
> Hmm, the test case I used was 'perf top'.  This was only using perf
> counter0.  So I am not sure, at least in my case, it was a stale event.
> Though I don't think I remember checking the status immediately after
> acking it just to verify the ack worked.

I am not acking it right away, this is still without patch 4/4. I am trying to
understand how we can get to this situation.

>
> I'll poke some more on my machine today.
>
> Cheers,
> Don
>
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