linux-kernel - Re: [PATCH 05/10] x86

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Message-Id: <200712290255.40233.ak@suse.de>
Date:	Sat, 29 Dec 2007 02:55:39 +0100
From:	Andi Kleen <ak@...e.de>
To:	Mike Travis <travis@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>, tglx@...utronix.de,
	mingo@...hat.com
Subject: Re: [PATCH 05/10] x86_64: Use generic percpu

On Friday 28 December 2007 23:05:05 Mike Travis wrote:
> Andi Kleen wrote:
> > On Friday 28 December 2007 01:10:51 travis@....com wrote:
> >> x86_64 provides an optimized way to determine the local per cpu area
> >> offset through the pda and determines the base by accessing a remote
> >> pda.
> >
> > And? The rationale for this patch seems to be incomplete.
> >
> > As far as I can figure out you're replacing an optimized percpu
> > implementation which a dumber generic one. Which needs
> > at least some description why.
> 
> The specific intent for the next wave of changes coming are to reduce
[...] That should be in the changelog of the patch.

Anyways the difference between the x86 percpu.h and the generic one is
that x86-64 uses a short cut through the PDA to get the current cpu
offset for the current CPU case. The generic one goes through 
smp_processor_id()->array reference instead. 

I would request that this optimization is not being removed
without suitable replacement in the same patchkit.

-Andi
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