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

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Message-ID: <20080814180943.GD4697@Krystal>
Date:	Thu, 14 Aug 2008 14:09:43 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

* H. Peter Anvin (hpa@...or.com) wrote:
> Mathieu Desnoyers wrote:
>> Sure, here are the updated tables. Basically, they show no significant
>> difference between the NOP and the DS segment selector prefix
>> approaches.
>
> Actually, unless I have blown my T-test completely, they show with a 80% 
> and 74% confidence (respective for the two benchmarks) that the DS case is 
> slightly *better* (0.26% and 0.20% better, respective), which makes it a 
> no-brainer.  Doing around 10 runs of each is likely to confirm this 
> conclusion by pushing it into the 90+% interval.
>

Yes, I think you are right.

> Note that since the difference is so small, and so can also be due to some 
> kind of systematic error (lower ambient temperature during the DS run 
> making the disk drive slightly faster, what have you.)
>

I doubt it, because I made a "cache priming" run before the tests, which
made sure the data was all populated in memory. But yeah, having
different cpu clock calibration values/cpu clock frequency between
reboots could also cause that kind of difference.

Mathieu

> 	-hpa

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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