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 20:28:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...hat.com,
	hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	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


* Ingo Molnar <mingo@...e.hu> wrote:

> The GUP based method is pretty generic though - and can be used on 
> other architectures as well. It's not as fast as direct access 
> though.

Another question is: your patch switches over all normal exceptions 
from IRET to hand-unroll+RET.

It would be really nice to benchmark it (via 'perf stat' for example 
;-) whether that's a slowdown or a speedup.

If it's a slowdown then the decision is easy: we dont want this, we 
want to push the overhead into the sampling code, away from common 
codepaths.

[ If on the other hand it's a speedup of a few cycles then we have 
  the problem of me suddenly liking this patch a whole lot more ;-) ]

	Ingo
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