linux-kernel - Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug

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Date:	Thu, 14 Aug 2008 10:05:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with	preemptible
 kernel and CPU hotplug

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.com) wrote:
>   
>> Mathieu Desnoyers wrote:
>>     
>>> I can't argue about the benefit of using VM CPU pinning to manage
>>> resources because I don't use it myself, but I ran some tests out of
>>> curiosity to find if uncontended locks were that cheap, and it turns out
>>> they aren't. Here are the results :
>>> Xeon 2.0GHz
>>> Summary
>>> make -j1 kernel/      33.94 +/- 0.07         34.91 +/- 0.27      2.8 %
>>> hackbench 50           2.99 +/- 0.01          3.74 +/- 0.01     25.1 %
>>> 1 CPU, replace smp lock prefixes with DS segment selector prefixes
>>> 1 CPU, noreplace-smp
>>>       
>> For reference, could you also compare replace smp lock with NOPs?
>>
>> 	-hpa
>>     
>
> Sure, here are the updated tables. Basically, they show no significant
> difference between the NOP and the DS segment selector prefix
> approaches.
>   

BTW, are you changing the initial prefix to DS too?  Ie, are you doing a 
nop->lock->ds transition, or ds->lock->ds?

    J
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