linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Date:	Mon, 15 Jun 2009 21:03:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	vegard.nossum@...il.com, efault@....de, jeremy@...p.org,
	npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Peter Zijlstra (a.p.zijlstra@...llo.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
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