linux-kernel - Re: [PATCH 0/3] percpu: Optimize percpu accesses

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Date:	Thu, 24 Jan 2008 23:46:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	travis@....com
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Andi Kleen <ak@...e.de>,
	Christoph Lameter <clameter@....com>, jeremy@...p.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] percpu: Optimize percpu accesses


* travis@....com <travis@....com> wrote:

> This patchset provides the following:
> 
>   * Generic: Percpu infrastructure to rebase the per cpu area to zero
> 
>     This provides for the capability of accessing the percpu variables
>     using a local register instead of having to go through a table
>     on node 0 to find the cpu-specific offsets.  It also would allow
>     atomic operations on percpu variables to reduce required locking.
> 
>   * x86_64: Fold pda into per cpu area
> 
>     Declare the pda as a per cpu variable. This will move the pda
>     area to an address accessible by the x86_64 per cpu macros.
>     Subtraction of __per_cpu_start will make the offset based from
>     the beginning of the per cpu area.  Since %gs is pointing to the
>     pda, it will then also point to the per cpu variables and can be
>     accessed thusly:
> 
> 	%gs:[&per_cpu_xxxx - __per_cpu_start]
>
>   * x86_64: Rebase per cpu variables to zero
> 
>     Take advantage of the zero-based per cpu area provided above. Then 
>     we can directly use the x86_32 percpu operations. x86_32 offsets 
>     %fs by __per_cpu_start. x86_64 has %gs pointing directly to the 
>     pda and the per cpu area thereby allowing access to the pda with 
>     the x86_64 pda operations and access to the per cpu variables 
>     using x86_32 percpu operations.

tried it on x86.git and 1/3 did not build and 2/3 causes a boot hang 
with the attached .config.

	Ingo

View attachment "config" of type "text/plain" (71883 bytes)