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

From: Eric Dumazet
Date: Sat May 17 2008 - 03:37:21 EST

Rusty Russell a écrit :
On Friday 16 May 2008 23:41:16 Mike Travis wrote:
Eric Dumazet wrote:
Rusty Russell a écrit :
Any chance I can ask you look at the issue of full dynamic per-cpu
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

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:

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