Re: [PATCH v7] sched: fix init NOHZ_IDLE flag

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2013-04-23 at 09:52 +0200, Vincent Guittot wrote:

> > static inline unsigned long *rq_nohz_flags(int cpu)
> > {
> >         return rcu_dereference(cpu_rq(cpu)->sd)->nohz_flags;
> > }
> >
> >         if (!test_bit(0, rq_nohz_flags(cpu)))
> >                 return;
> >         clear_bit(0, rq_nohz_flags(cpu));
> >
> 
> AFAICT, if we use different rcu_dereferences for modifying nohz_flags
> and for updating the nr_busy_cpus, we open a time window for
> desynchronization, isn't it ?

Oh right, we need to call rq_nohz_flags() once and use the pointer
twice.

> >> +unlock:
> >>       rcu_read_unlock();
> >>  }
> >>
> >> @@ -5409,14 +5416,21 @@ void set_cpu_sd_state_idle(void)
> >>  {
> >>       struct sched_domain *sd;
> >>       int cpu = smp_processor_id();
> >> -
> >> -     if (test_bit(NOHZ_IDLE, nohz_flags(cpu)))
> >> -             return;
> >> -     set_bit(NOHZ_IDLE, nohz_flags(cpu));
> >> +     int first_nohz_idle = 1;
> >>
> >>       rcu_read_lock();
> >> -     for_each_domain(cpu, sd)
> >> +     for_each_domain(cpu, sd) {
> >> +             if (first_nohz_idle) {
> >> +                     if (test_bit(NOHZ_IDLE, &sd->nohz_flags))
> >> +                             goto unlock;
> >> +
> >> +                     set_bit(NOHZ_IDLE, &sd->nohz_flags);
> >> +                     first_nohz_idle = 0;
> >> +             }
> >> +
> >>               atomic_dec(&sd->groups->sgp->nr_busy_cpus);
> >> +     }
> >> +unlock:
> >
> > Same here, .. why on earth do it for every sched_domain for that
> cpu?
> 
> The update of rq_nohz_flags is done only once on the top level
> sched_domain and the nr_busy_cpus is updated on each sched_domain
> level in order to have a quick access to the number of busy cpu when
> we check for the need to kick an idle load balance

Ah, I didn't see the whole first_nohz_idle thing.. but why did you
place it inside the loop in the first place? Wouldn't GCC be able to
avoid the double cpu_rq(cpu)->sd dereference using CSE? Argh no,.. its
got an ACCESS_ONCE in it that defeats GCC.

Bothersome..

I'd almost write it like:

	struct sched_domain *sd;

	rcu_read_lock();
	sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd);

	if (sd->nohz_idle)
		goto unlock;
	xchg(&sd->nohz_idle, 1); /* do we actually need atomics here? */

	for (; sd; sd = sd->parent)
		atomic_dec(&sd->groups->sgp->nr_busy_cpus);
  unlock:
	rcu_read_unlock();