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

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Message-ID: <48764840.6010607@linux-foundation.org>
Date:	Thu, 10 Jul 2008 12:34:56 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mike Travis <travis@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.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>,
	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

Jeremy Fitzhardinge wrote:
> Christoph Lameter wrote:
>> Jeremy Fitzhardinge wrote:
>>
>>  
>>> You want to virtually map the percpu area?  How and when would it get
>>> extended?
>>>     
>>
>> It would get extended when cpu_alloc() is called and the allocator
>> finds that there is no per cpu memory available.
>>   
> 
> Which, I take it, allocates percpu memory.  It would have the same
> caveats as vmalloc memory, with respect to accessing it during fault
> handlers and nmi handlers, I take it.

Right. One would not want to allocate per cpu memory in those contexts. The current allocpercpu() functions already have those restrictions.

> How would cpu_alloc() actually get used?  It doesn't make much sense for
> general code, since we don't have the notion of a percpu pointer to
> memory (vs a pointer to percpu memory).  Is the intended use for
> allocating percpu memory in modules?  What other uses?

Argh. Do I have to reexplain all of this all over again? Please look at the latest cpu_alloc patchset and the related discussions.

The cpu_alloc patchset introduces the concept of a pointer to percpu memory. Or you could call it an offset into the percpu segment that can be treated (in a restricted way) like a pointer....



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