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

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Message-ID: <4874F7D9.5060607@linux-foundation.org>
Date:	Wed, 09 Jul 2008 12:39:37 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mike Travis <travis@....com>, 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

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.

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