linux-kernel - Re: [PATCH] modules: Use a better scheme for refcounting

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Message-ID: <482E8AF5.9050804@cosmosbay.com>
Date:	Sat, 17 May 2008 09:36:21 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] modules: Use a better scheme for refcounting

Rusty Russell a écrit :
> On Friday 16 May 2008 23:41:16 Mike Travis wrote:
>   
>> Eric Dumazet wrote:
>>     
>>> Rusty Russell a Ã©crit :
>>>       
>>>>    Any chance I can ask you look at the issue of full dynamic per-cpu
>>>> allocation?
>>>>         
>>> You mean using alloc_percpu() ? Problem is that current implementation
>>> is expensive,
>>>       
>
> I mean rewriting alloc_percpu :)
>
>   
>>> We probably can change this to dynamic per-cpu as soon as Mike or
>>> Christopher finish their work on new dynamic per-cpu implementation ?
>>>       
>> Yes, the zero-based percpu variables followed by the cpu_alloc patch should
>> provide this and shrink the code quite well, including in some cases
>> removing locking requirements (because the resultant instructions will be
>> atomic.)
>>     
>
> Ah, I hadn't realized that Mike was already working on this.  Mike, have you 
> published patches already?
>
>   
Christoph Lameter made good work last year and apparently the path is to :

1) Put pda at the begining of percpu section.
2) Relocate percpu variables to begin at zero
3) Use %gs (or %fs) register to address pda AND percpu section.
4) Reserve an appropriate large virtual zone into percpu section so that 
it can fullfill :
- percpu "static" allocations needed by module loader.
- per_cpu "dynamic" allocations
  (No more need for an array of pointers to find the address of dynamic 
per_cpu variable)
- Eventually allocates PAGES dynamically into this zone to satisfy any 
size of percpu needs.
(But with a limit of say 256 MB per cpu on x86_64)

Thats a lot of work and apparently Mike and Christoph are doing it.

Some pointers to previous work:

http://groups.google.pl/group/linux.kernel/browse_thread/thread/f2ff6901ca6ae9fc/b2ed3b7f3612a157?lnk=raot

http://kerneltrap.org/mailarchive/linux-kernel/2008/2/1/683104

http://linux.derkeiler.com/Mailing-Lists/Kernel/2008-01/msg09052.html




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