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

Eric Dumazet
2008-05-17T07:36:50Z

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: