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Date: Mon, 15 Jun 2009 21:03:21 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, "H. Peter Anvin" <hpa@zytor.com>,
       Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
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       linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods
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* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:
> > On Mon, 2009-06-15 at 14:48 -0400, Mathieu Desnoyers wrote:
> > > we should not care that much about the performance hit of
> > > saving/restoring the cr2 register at each nmi entry/exit.
> > 
> > But we do, perf counters very much cares about nmi performance.
> > 
> 
> To a point where it cannot afford a simple register save/restore ?
> 
> There is "caring" and "_caring_". I am tempted to ask what NMI 
> handler execution frequency you have in mind here to figure out if 
> we are not trying to optimize sub-nanoseconds per minutes. ;)

I routinely run 'perf' with half a million NMIs per second or more. 
( Why wait 10 seconds for a profile you can get in 1 second? ;-)

Granted that is over multiple CPUs - but still performance does 
matter here too.

Reading cr2 is certainly fast. Writing it - dunno.

	Ingo