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Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
 support to use NMI-safe methods
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
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Date: Mon, 15 Jun 2009 21:10:11 +0200
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On Mon, 2009-06-15 at 21:07 +0200, Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> > > 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.
> 
> But one thing is sure: it is certainly going to be faster than the 
> INVLPG(s!) we have to do with the GUP solution.

Sure, but we only pay that price when we do the callchain bit, not on
every NMI.