linux-kernel - Re: [PATCH 3/4] Pte xchg optimization.patch

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Message-ID: <461E7E0A.60002@vmware.com>
Date:	Thu, 12 Apr 2007 11:44:26 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Dave Jones <davej@...hat.com>, Zachary Amsden <zach@...are.com>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@....de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Hugh Dickins <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Michel Lespinasse <walken@...are.com>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] Pte xchg optimization.patch

Dave Jones wrote:
> On Wed, Apr 11, 2007 at 10:30:58PM -0700, Zachary Amsden wrote:
>  > In situations where page table updates need only be made locally, and there
>  > is no cross-processor A/D bit races involved, we need not use the heavyweight
>  > xchg instruction to atomically fetch and clear page table entries.  Instead,
>  > we can just read and clear them directly.
>  > 
>  > This introduces a neat optimization for non-SMP kernels; drop the atomic
>  > xchg operations from page table updates.
>
> Would it be feasible to do this using the smp_alternatives stuff?
> Given many distros are now shipping SMP-only kernels, it'd be nice
> to get that win without using having to rebuild their kernels.
>   

That's a good point. It is a fairly straightforward substitution for the 
static case:

xchg reg, mem

to

mov mem, reg
mov imm, mem

I'll give it a spin and see how it drives. It is, though, less obvious 
how easy it will be for PAE kernels.

Zach
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