Re: [PATCH 13/32] nohz: Adaptive tick stop and restart on nohz cpuset

Frederic Weisbecker
2011-08-29T18:28:45Z

On Mon, Aug 29, 2011 at 08:07:10PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-29 at 20:02 +0200, Frederic Weisbecker wrote:
> > On Mon, Aug 29, 2011 at 05:28:09PM +0200, Peter Zijlstra wrote:
> > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > > > +bool cpuset_nohz_can_stop_tick(void)
> > > > +{
> > > > + struct rq *rq;
> > > > +
> > > > + rq = this_rq();
> > > > +
> > > > + /* More than one running task need preemption */
> > > > + if (rq->nr_running > 1)
> > > > + return false;
> > > > +
> > > > + return true;
> > > > +}
> > >
> > > int sched_needs_cpu(int cpu), seems the right name, matches the existing
> > > {rcu,printk,arch}_needs_cpu() functions.
> >
> > tick_nohz_stop_sched_tick() already handles that by keeping a periodic
> > behaviour if one of these conditions are met.
>
> What? tick_nohz_stop_sched_tick() most surely cannot access struct rq,
> so it cannot do the nr_running test.

I was talking about {rcu,printk,arch}_needs_cpu() functions.

>
> > It has also the upside to restore the periodic behaviour if needed
> > from irq return if the tick was stopped.
>
> Again, what?

If the tick is stopped then an irq fires and something calls printk() or call_rcu()
then on interrupt return, tick_nohz_stop_sched_tick() checks that
with {rcu,printk,arch}_needs_cpu() and restores a periodic behaviour
until nobody else needs the CPU.