linux-kernel - Re: [cpuops cmpxchg V1 2/4] x86: this_cpu_cmpxchg and this_cpu

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Message-ID: <4D0076B3.9090106@zytor.com>
Date:	Wed, 08 Dec 2010 22:26:59 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [cpuops cmpxchg V1 2/4] x86: this_cpu_cmpxchg and	this_cpu_xchg
 operations

On 12/08/2010 10:17 AM, Mathieu Desnoyers wrote:
> 
> Hi Christoph,
> 
> Can you show if this provides savings in terms of:
> 
> - instruction cache footprint
> - cycles required to run
> - large-scale impact on the branch prediction buffers
> 
> Given that this targets per-cpu data only, the additional impact on cache-line
> exchange traffic of using cmpxchg over xchg (cache-line not grabbed as exclusive
> by the initial read) should not really matter.
> 
> I'm CCing Arjan and HPA, because they might have some interesting insight into
> the performance impact of lock-prefixed xchg vs using local cmpxchg in a loop.
> 

XCHG is always locked; it doesn't need the prefix.  Unfortunately,
unlike on the 8086 on modern processors locks have a real cost.

	-hpa

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

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