linux-kernel - Re: A question of perf NMI handler

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Date:	Wed, 04 Aug 2010 17:02:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Robert Richter <robert.richter@....com>,
	Lin Ming <ming.m.lin@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>
Subject: Re: A question of perf NMI handler

On Wed, 2010-08-04 at 10:52 -0400, Don Zickus wrote:
> > Right so I looked up your thing and while that limits the damage in that
> > at some point it will let NMIs pass, it will still consume too many.
> > Meaning that Yinghai will have to potentially press his NMI button
> > several times before it registers.
> 
> Ok.  Thanks for reviewing.  How does it consume to many?  I probably don't
> understand how perf is being used in the non-simple scenarios.

Suppose you have 4 counters (AMD, intel-nhm+), when more than 2 overflow
the first will raise the PMI, if the other 2+ overflow before we disable
the PMU it will try to raise 2+ more PMIs, but because hardware only has
a single interrupt pending bit it will at most cause a single extra
interrupt after we finish servicing the first one.

So then the first interrupt will see 3+ overflows, return 3+, and will
thus eat 2+ NMIs, only one of which will be the pending interrupt,
leaving 1+ NMIs from other sources to consume unhandled.

In which case Yinghai will have to press his NMI button 2+ times before
it registers.

That said, that might be a better situation than always consuming
unknown NMIs.. 
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