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

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Message-ID: <477916A9.6030001@sgi.com>
Date:	Mon, 31 Dec 2007 08:19:53 -0800
From:	Mike Travis <travis@....com>
To:	Andi Kleen <ak@...e.de>
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

Andi Kleen wrote:
> 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

I'll take a closer look at this, but it may have to wait until Christoph
is back this Wednesday.

Thanks,
Mike
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