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 21:59:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"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


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Mon, 2009-06-15 at 21:07 +0200, Ingo Molnar wrote:
> > * Ingo Molnar <mingo@...e.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.

same goes for the CR2 save/restore trick - it only has to be done 
around the code where we expect to generate a #PF. I.e. in the 
call-graph bits.

	Ingo
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