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

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Message-ID: <4874FCA7.3010507@sgi.com>
Date:	Wed, 09 Jul 2008 11:00:07 -0700
From:	Mike Travis <travis@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

Jeremy Fitzhardinge wrote:
> 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

I did compartmentalize the changes so they were in separate patches,
and in particular, by separating the changes to the include files, I
was able to zero in on some problems much easier.

But I have no objections to leaving the cpu_pda ops in place and then,
as you're suggesting, extract and modify the fields as appropriate.

Another approach would be to leave the changes from XXX_pda() to
x86_percpu_XXX in place, and do the patches with simply changing
pda.VAR to VAR .)

In any case I would like to get this version working first, before
attempting that rewrite, as that won't change the generated code.

Btw, while I've got your attention... ;-), there's some code in
arch/x86/xen/smp.c:xen_smp_prepare_boot_cpu() that should be looked
at closer for zero-based per_cpu__gdt_page:

	make_lowmem_page_readwrite(&per_cpu__gdt_page);

(I wasn't sure how to deal with this but I suspect the __percpu_offset[]
or __per_cpu_load should be added to it.)

Thanks,
Mike
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