linux-kernel - Re: [PATCH -v3] perf, x86: try to handle unknown nmis with running perfctrs

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Date:	Fri, 27 Aug 2010 09:44:29 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Robert Richter <robert.richter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Lin Ming <ming.m.lin@...el.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH -v3] perf, x86: try to handle unknown nmis with running
 perfctrs

On Fri, Aug 27, 2010 at 10:10:38AM +0200, Robert Richter wrote:
> On 26.08.10 17:14:24, Don Zickus wrote:
> > diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> > index 4539b4b..d16ebd8 100644
> > --- a/arch/x86/kernel/cpu/perf_event_intel.c
> > +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> > @@ -738,6 +738,7 @@ again:
> >  
> >  	inc_irq_stat(apic_perf_irqs);
> >  	ack = status;
> > +	intel_pmu_ack_status(ack);
> 
> I would slightly change the patch:
> 
> There is no need for the ack variable anymore, you could directly work
> with the status.
> 
> I would call intel_pmu_ack_status() as close as possible after the
> intel_pmu_get_status(), which is after 'again:'.

Yeah, I can do that.  The other patch was just a proof of concept to see
what others thought.

What is funny is that this problem was masked by the
perf_event_nmi_handler swallowing all the nmis.  I wonder if we were
losing events as a result of this bug too because if you think about it,
we processed the first event, a second event came in and we accidentally
ack'd it, thus dropping it on the floor.  Now I wonder how the event was
ever reloaded, unless it was by accident because of how the scheduler
deals with perf counters (perf_start/stop all the time).

Cheers,
Don
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