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SubjectRe: tree rcu: call_rcu scalability problem?
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On Wed, 2009-09-02 at 14:27 +0200, Nick Piggin wrote:

> It seems like nearly 2/3 of the cost is here:
> /* Add the callback to our list. */
> *rdp->nxttail[RCU_NEXT_TAIL] = head; <<<
> rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
>
> In loading the pointer to the next tail pointer. If I'm reading the profile
> correctly. Can't see why that should be a probem though...
>
> ffffffff8107dee0 <__call_rcu>: /* __call_rcu total: 320971 100.000 */
> 697 0.2172 :ffffffff8107dee0: push %r12

> 921 0.2869 :ffffffff8107df57: push %rdx
> 151 0.0470 :ffffffff8107df58: popfq
> 183507 57.1725 :ffffffff8107df59: mov 0x50(%rbx),%rax
> 995 0.3100 :ffffffff8107df5d: mov %rdi,(%rax)

I'd guess at popfq to be the expensive op here.. skid usually causes the
attribution to be a few ops down the line.



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