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

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Message-Id: <1245092561.6741.205.camel@laptop>
Date:	Mon, 15 Jun 2009 21:02:41 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, 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

On Mon, 2009-06-15 at 14:59 -0400, Mathieu Desnoyers 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. ;)

Ah, well, I have no idea who expensive cr2 is, if its like a regular
register then it should be fine. If however its tons more expensive then
we should really avoid it.

As to the freq, 100kHz would be nice ;-)

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