Re: SMP syncronization on AMD processors (broken?)

[Eric Dumazet]

Andi Kleen a écrit :
> Kirill Korotaev <dev@xxxxx> writes:
>
>
> > Please help with a not simple question about spin_lock/spin_unlock on
> > SMP archs. The question is whether concurrent spin_lock()'s should
> > acquire it in more or less "fair" fashinon or one of CPUs can starve
> > any arbitrary time while others do reacquire it in a loop.
>
>
> They are not fully fair because of the NUMAness of the system.
> Same on many other NUMA systems.
>
> We considered long ago to use queued locks to avoid this, but 
> they are quite costly for the uncongested case and never seemed worth it.
>
> So live with it.

Unrelated, but that reminds me that current spinlock implementation on x86 
imply that NR_CPUS should be < 128.

Maybe we should reflect this in Kconfig ?

config NR_CPUS
range 2 128

Or use a plain int for spinlock, instead of a signed char.

Eric
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