linux-kernel - Re: [PATCH 4/7] perf, x86: Implement IBS interrupt handler

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Date:	Fri, 5 Aug 2011 09:47:48 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] perf, x86: Implement IBS interrupt handler

On Fri, Aug 05, 2011 at 11:55:19AM +0200, Ingo Molnar wrote:
> > I tried looking into but everytime I applied workarounds for Intel 
> > errata I wound up with more unknown NMIs and proving that a couple 
> > of them worked (with trace_printks) seemed elusive.  I got 
> > frustrated and left it alone.
> > 
> > But yeah, Intel's perf has so many errata that I think if you kick 
> > the box while running perf you can generate an unknown NMI.
> 
> Hence the only sane approach is to just tolerate spurious NMIs and 
> only annoy the user with them if there's *way* too many of them or 
> so.

That may work if we can determine if the user is running perf or not.  But
on older systems (like pre-Nehalem), sometimes the only way a system can
signal a platform error is through a single unknown NMI.  I would be
afraid we might lose one of those if we 'tolerate' unknown NMIs.

So far I have only noticed perf generating 'unknown NMIs' on high volume
usage (like multiple counters).  For the casual user it has been ok so
far.

Cheers,
Don
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