Re: [PATCH RESEND v9 10/10] sched: move cfs task on a CPU with higher capacity

Peter Zijlstra
2015-02-20T11:27:52Z

On Thu, Jan 15, 2015 at 11:09:30AM +0100, Vincent Guittot wrote:
> As a sidenote, this will note generate more spurious ilb because we already

s/note/not/

> trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that
> has a task, we will trig the ilb once for migrating the task.

> +static inline bool nohz_kick_needed(struct rq *rq)
> {
> unsigned long now = jiffies;
> struct sched_domain *sd;
> struct sched_group_capacity *sgc;
> int nr_busy, cpu = rq->cpu;
> + bool kick = false;
>
> if (unlikely(rq->idle_balance))
> + return false;
>
> /*
> * We may be recently in ticked or tickless idle mode. At the first
> @@ -7472,38 +7498,44 @@ static inline int nohz_kick_needed(struct rq *rq)
> * balancing.
> */
> if (likely(!atomic_read(&nohz.nr_cpus)))
> + return false;
>
> if (time_before(now, nohz.next_balance))
> + return false;
>
> if (rq->nr_running >= 2)
> + return true;

So this,

> rcu_read_lock();
> sd = rcu_dereference(per_cpu(sd_busy, cpu));
> if (sd) {
> sgc = sd->groups->sgc;
> nr_busy = atomic_read(&sgc->nr_busy_cpus);
>
> + if (nr_busy > 1) {
> + kick = true;
> + goto unlock;
> + }
> +
> }
>
> + sd = rcu_dereference(rq->sd);
> + if (sd) {
> + if ((rq->cfs.h_nr_running >= 1) &&
> + check_cpu_capacity(rq, sd)) {
> + kick = true;
> + goto unlock;
> + }
> + }

vs this: how would we ever get here?

If h_nr_running > 1, must then not nr_running > 1 as well?

>
> + sd = rcu_dereference(per_cpu(sd_asym, cpu));
> if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
> sched_domain_span(sd)) < cpu))
> + kick = true;

For consistencies sake I would've added a goto unlock here as well.

> +unlock:
> rcu_read_unlock();
> + return kick;
> }