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

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Date:	Fri, 27 Aug 2010 11:05:23 -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 04:05:23PM +0200, Robert Richter wrote:
> > 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.
> 
> Yes, this could be the case, but only for handled counters. So it
> would be interesting to see for this case the status mask of the
> current and previous get_status call.

The status masks seem to be identical, 0x1 (and when I forced pmc0
unusable, everything was 0x2).

> 
> > 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).
> 
> The nmi might be queued be the cpu regardless of of the overflow
> state.
> 
> I am wondering why this happens at all, because events are disabled by
> wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, 0). Hmm, maybe this is exactly the

Heh.  Not sure why it isn't working then.  Then again you shouldn't need
the loop if it was working I would think.

> reason because the nmi could fire again after reenabling the counters.
> 
> Is there a reason for disabling all counters?

It would be a nice to have that way we wouldn't have to 'eat' all these
extra nmis.  But I guess it isn't working correctly.

Cheers,
Don
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