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

[Paul E. McKenney]

On Mon, Aug 25, 2008 at 10:46:06AM -0500, Christoph Lameter wrote:
> 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.

The problem is that we need dec_and_zero on the sum of the per-CPU
counters.  Gets spendy.  One can make a hierarchy, and propagate up.
But still lots of cache misses.

							Thanx, Paul
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