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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Aug 25, 2008 at 05:51:32PM +0200, Peter Zijlstra wrote:
> On Mon, 2008-08-25 at 10:46 -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.
> 
> Hmm, perhaps that might work for classic RCU, as that disables
> preemption and thus the counters should always be balanced.

Unless you use a pair of global counters (like QRCU), you will still
need to check a large number of counters for zero.  I suppose that one
approach would be to do something like QRCU, but with some smallish
number of counter pairs, each of which is shared by a moderate group of
CPUs.  For example, for 4,096 CPUs, use 64 pairs of counters, each
shared by 64 CPUs.  My guess is that the rcu_read_lock() overhead would
make this be a case of "Holy overhead, Batman!!!", but then again, I
cannot claim to be an expert on 4,096-CPU machines.

							Thanx, Paul