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

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Date:	Wed, 09 Jul 2008 10:27:14 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mike Travis <travis@....com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Mike Travis wrote:
> This patchset provides the following:
>
>   * Cleanup: Fix early references to cpumask_of_cpu(0)
>
>     Provides an early cpumask_of_cpu(0) usable before the cpumask_of_cpu_map
>     is allocated and initialized.
>
>   * 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.
>     Uses a new config var HAVE_ZERO_BASED_PER_CPU to indicate to the
>     generic code that the arch has this new basing.
>
>     (Note: split into two patches, one to rebase percpu variables at 0,
>     and the second to actually use %gs as the base for percpu variables.)
>
>   * 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.

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.

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.

    J
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