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

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Message-ID: <48A465BA.5050406@goop.org>
Date:	Thu, 14 Aug 2008 10:04:58 -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:
> 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 :
>   

OK, let me clarify my point a bit.  If you've got a kernel which is 
switching between UP and SMP on a regular basis, you're going to be 
taking the hit for the locked instructions whenever you're in the SMP 
state anyway.  It's only going to be a workload where you're mostly UP 
with occasional excursions into being SMP that patching out the lock 
prefixes is actually going to make a difference. 

And that just doesn't seem like a very likely use-case to me.  Certainly 
I don't think it would ever happen on a physical machine.  And it 
doesn't seem all that likely on a virtual machine either. Certainly 
resources are more dynamic in a virtual environment, but I think there's 
a fairly good chance that the domain knows from the outset whether it's 
going to be UP or SMP, or does the UP->SMP transition once.

(That said, the XenServer product does precisely what I say is unusual: 
the dom0 kernel hotplugs all the cpus so it can do ucode updates, etc, 
and then unplugs all but one...)

> Xeon 2.0GHz
>
>
> Summary
>
>                   no lock prefix (s)   with lock prefix (s)    Speedup
> 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 %
>   

Yeah, that's more severe than I would have expected.  Perhaps I have AMD 
numbers in my head.

    J
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