linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Wed, 09 Jul 2008 11:02:58 -0700
From:	Mike Travis <travis@....com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jack Steiner <steiner@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Christoph Lameter wrote:
> Jeremy Fitzhardinge wrote:
> 
>> The bulk of this series is pda_X to x86_X_percpu conversion.  This seems
>> like pointless churn to me; there's nothing inherently wrong with the
>> pda_X interfaces, and doing this transformation doesn't get us any
>> closer to unifying 32 and 64 bit.
> 
> What is the point of the pda_X interface? It does not exist on 32 bit.
> The pda wastes the GS segment register on a small memory area. This patchset
> makes the GS segment usable to reach all of the per cpu area by placing
> the pda into the per cpu area. Thus the pda_X interface becomes obsolete
> and the 32 bit per cpu stuff becomes usable under 64 bit unifying both
> architectures.
> 
> 
>> I think we should start devolving things out of the pda in the other
>> direction: make a series where each patch takes a member of struct
>> x8664_pda, converts it to a per-cpu variable (where possible, the same
>> one that 32-bit uses), and updates all the references accordingly.  When
>> the pda is as empty as it can be, we can look at removing the
>> pda-specific interfaces.
> 
> This patchset places the whole x8664_pda structure into the per cpu area and makes the pda macros operate on the x8664_pda structure in the per cpu area. Not sure why you want to go through the churn of doing it for each object separately.

I think Jeremy's point is that by removing the pda struct entirely, the
references to the fields can be the same for both x86_32 and x86_64.

Thanks,
Mike
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