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

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Date:	Thu, 10 Jul 2008 12:56:23 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>   
>> Eric W. Biederman wrote:
>>     
>>> I think we can get away with just simply realloc'ing the percpu area
>>> on each cpu.  No fancy table manipulations required.  Just update
>>> the base pointer in %gs and in someplace global.
>>>
>>>       
>> It's perfectly legitimate to take the address of a percpu variable and store it
>> somewhere.  We can't move them around.
>>     
>
> Really?  I guess there are cases where that makes sense.  It is a pretty
> rare case though.  Especially when you are not talking about doing it temporarily
> with preemption disabled.  There are few enough users of the API I think we can
> certainly explore the cost of forbidding in the general case of storing the
> address of a percpu variable.
>   

No, that sounds like a bad idea.  For one, how would you enforce it?  
How would you check for it?  It's one of those things that would mostly 
work and then fail very rarely.

Secondly, I depend on it.  I register a percpu structure with Xen to 
share per-vcpu specific information (interrupt mask, time info, runstate 
stats, etc).

    J
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