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

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Message-ID: <CA+55aFz3S6_g4TQ11VKGM3GW=a=r3qYWQ_a0qJ_5241qQz8D=w@mail.gmail.com>
Date:	Tue, 10 Sep 2013 09:24:21 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.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 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?

           Linus
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