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

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Message-ID: <alpine.LFD.1.10.0808140901390.3324@nehalem.linux-foundation.org>
Date:	Thu, 14 Aug 2008 09:10:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.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

On Thu, 14 Aug 2008, 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.

Absolutely. 

Locked ops show up not just in microbenchmarks looping over the 
instruction, they show up in "real" benchmarks too. We added a single 
locked instruction (maybe it was two) to the page fault handling code some 
time ago, and the reason I noticed it was that it actually made the page 
fault cost visibly more expensive in lmbench. That was a _single_ 
instruction in the hot path (or maybe two).

And the page fault path is some of the most timing critical in the whole 
kernel - if you have everything cached, the cost of doing the page faults 
to populate new processes for some fork/exec-heavy workload (and compiling 
the kernel is just one of those - any traditional unix behaviour will show 
this) is critical.

This is one of the things AMD does a _lot_ better than Intel. Intel tends 
to have a 30-50 cycle cost (with later P4s being *much* worse), while AMD 
tends to have a cost of around 10-15 cycles.

It's one of the things Intel promises to have improved in the next-gen 
uarch (Nehalem), an while I am not supposed to give out any benchmarks, I 
can confirm that Intel is getting much better at it. But it's going to be 
visible still, and it's really a _big_ issue on P4.

(Of course, on P4, the page fault exception cost itself is so high that 
the cost of atomics may be _relatively_ less noticeable in that particular 
path)

		Linus
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