linux-kernel - Re: [PATCH 0/7] preempt

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Message-ID: <5230939E.3050408@zytor.com>
Date:	Wed, 11 Sep 2013 09:00:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Arjan van de Ven <arjan@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 0/7] preempt_count rework -v2

On 09/10/2013 09:24 AM, Linus Torvalds wrote:
> On Tue, Sep 10, 2013 at 8:29 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>>
>> also.. yuck on using "dec"
>> "dec" sucks, please use "sub foo  ,1" instead
> 
> That's a bigger instruction, largely due to the constant.
> 
>> (dec sucks because of its broken flags behavior; it creates basically a
>> bubble in the pipeline)
> 
> Intel could (and should) just fix that. It's "easy" enough - you just
> need to rename the carry flag as a separate register (and make sure
> that the conditional instructions that don't use it don't think they
> need it).
> 
> In fact I thought Intel already did that on their large cores. Are you
> sure they still have trouble with inc/dec?
> 

Big cores are fine, but Atom (and presumably Quark) might still suffer a
performance penalty.

	-hpa


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