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

[Eric Dumazet]

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
> > > > 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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