linux-kernel - Re: [PATCH 1/2] Define new percpu interface for shared data -- version 3

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Date:	Wed, 23 May 2007 10:49:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Fenghua Yu <fenghua.yu@...el.com>
Cc:	suresh.b.siddha@...el.com, clameter@....com, kiran@...lex86.org,
	rmk@....linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Define new percpu interface for shared data --
 version 3

On Tue, 22 May 2007 11:20:03 -0700 Fenghua Yu <fenghua.yu@...el.com> wrote:

> per cpu data section contains two types of data. One set which is exclusively
> accessed by the local cpu and the other set which is  per cpu, but also shared
> by remote cpus. In the current kernel, these two sets are not clearely
> separated out. This can potentially cause the same data cacheline shared
> between the two sets of data, which will result in unnecessary bouncing of the
> cacheline between cpus.
> 
> One way to fix the problem is to cacheline align the remotely accessed per cpu
> data, both at the beginning and at the end. Because of the padding at both ends,
> this will likely cause some memory wastage and also the interface to achieve
> this is not clean.
> 
> This patch:
> 
> Moves the remotely accessed per cpu data (which is currently marked
> as ____cacheline_aligned_in_smp) into a different section, where all the data
> elements are cacheline aligned. And as such, this differentiates the local
> only data and remotely accessed data cleanly.

OK, but could we please have a concise description of the impact
of these changes on kernel memory footprint?  Increase or decrease?
And by approximately how much?

Thanks.
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