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

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Message-ID: <4A369CD8.3090505@zytor.com>
Date:	Mon, 15 Jun 2009 12:11:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

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.

	-hpa

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