Re: [PATCH RFC tip/core/rcu 1/6] rcu: Stabilize use of num_online_cpus() for GP short circuit

[Rusty Russell <rusty@rustcorp.com.au>]

On Tue, 24 Apr 2012 09:50:56 -0700, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> On Tue, Apr 24, 2012 at 09:05:20PM +0530, Srivatsa S. Bhat wrote:
> 
> Thank you for looking this over!
> 
> > On 04/23/2012 10:12 PM, Paul E. McKenney wrote:
> > 
> > > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> > > 
> > > The rcu_blocking_is_gp() function tests to see if there is only one
> > > online CPU, and if so, synchronize_sched() and friends become no-ops.
> > > However, for larger systems, num_online_cpus() scans a large vector,
> > 
> > 
> > Venki had posted a patch to optimize that by using a variable, so that we
> > don't calculate the value each and every time.
> > http://thread.gmane.org/gmane.linux.kernel/1240569/focus=1246659
> > 
> > However, unfortunately there was some confusion around that patch and
> > even though it made it to akpm's tree and stayed there briefly, it didn't
> > go upstream. Venki had attempted to resolve the confusion here:
> > http://thread.gmane.org/gmane.linux.kernel/1240569/focus=1260702
> 
> Having a single variable tracking the online state would be good,
> but as you say it isn't there yet.

I think you could have a flag set by a notifier, but you'd have to
re-think the races...

Cheers,
Rusty.