linux-kernel - Re: Efficient x86 and x86

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Message-ID: <alpine.LFD.1.10.0808131320000.3462@nehalem.linux-foundation.org>
Date:	Wed, 13 Aug 2008 13:21:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: Efficient x86 and x86_64 NOP microbenchmarks



On Wed, 13 Aug 2008, Andi Kleen wrote:
> 
> Also I must say the whole ftrace noping exercise is pretty pointless without
> avoiding frame pointers because it does save less than what you lose
> unconditionally from the "select FRAME_POINTER"

Andi, you seem to have missed the whole point. This is a _correctness_ 
issue as long as the nop is not a single instruction. And the workaround 
for that is uglier than just making a single-instruction nop.

So the question now is to find a good nop that _is_ a single atomic 
instruction. Your blathering about frame pointers is missing the whole 
point!

			Linus
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