linux-kernel - Re: [cpuops cmpxchg V2 3/5] irq_work: Use per cpu atomics instead of regular atomics

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Message-ID: <4D08FB62.9070104@zytor.com>
Date:	Wed, 15 Dec 2010 09:31:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops cmpxchg V2 3/5] irq_work: Use per cpu atomics instead
 of regular atomics

On 12/15/2010 09:18 AM, Peter Zijlstra wrote:
> On Wed, 2010-12-15 at 11:04 -0600, Christoph Lameter wrote:
> 
>> Prefixes are faster than explicit address calculations. A prefix allows
>> you to integrate the per cpu address calculation into an arithmetic
>> operation.
> 
> Well, depends on how often you need that address I'd think. If you'd
> have a per-cpu struct and need to frob lots of variables in that struct
> it might be cheaper to simply compute the struct address once and then
> use relative addresses than to prefix everything with %fs.
> 

Let's just make it clear -- current x86 CPUs generally do not have a
penalty for prefixes (it might be that under very unusual pipeline
conditions they do, I am not 100% sure.)  In fact, we changed patching
LOCK prefixes from NOP to %ds: because it made the code faster.

Some older CPUs do, but those are no longer relevant for performance
decisions.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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