Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

[Christoph Lameter <cl@linux-foundation.org>]

Peter Zijlstra wrote:
>
> If we combine these two cases, and flip the counter as soon as we've
> enqueued one callback, unless we're already waiting for a grace period
> to end - which gives us a longer window to collect callbacks.
> 
> And then the rcu_read_unlock() can do:
> 
>   if (dec_and_zero(my_counter) && my_index == dying)
>     raise_softirq(RCU)
> 
> to fire off the callback stuff.
> 
> /me ponders - there must be something wrong with that...
> 
> Aaah, yes, the dec_and_zero is non trivial due to the fact that its a
> distributed counter. Bugger..

Then lets make it per cpu. If we get the cpu ops in then dec_and_zero would be
very cheap.