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 15:27:20 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

* H. Peter Anvin (hpa@...or.com) wrote:
> Peter Zijlstra wrote:
> > 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 ;-)
> > 
> 
> Writing control registers is serializing, so it's a lot more expensive
> than writing a normal register; my *guess* is that it will be on the
> order of 100-200 cycles.
> 
> That is not based on any actual information.
> 

Then how about just writing to the cr2 register *if* it has changed
while the NMI handler was running ?

if (unlikely(read_cr2() != saved_cr2)))
	write_cr2(saved_cr2)

Mathieu

> 	-hpa
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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