Re: [PATCH]sched_rt.c: Avoid unnecessary dequeue and enqueue of pushable tasks in set_cpus_allowed_rt()

Oleg Nesterov
2011-12-20T17:50:17Z

On 12/02, Kirill Tkhai wrote:
>
> Migration status depends on a difference of weight from 0 and 1. If
> weight > 1 (<= 1) and old weight <= 1 (> 1) then task becomes pushable
> (not pushable). We are not insterested in exact values of it, is it 3 or
> 4, for example.
>
> Now if we are changing affinity from a set of 3 cpus to a set of 4, the
> task will be dequeued and enqueued sequentially without important
> difference in comparison with initial state. The only difference is in
> internal representation of plist queue of pushable tasks and the fact
> that the task may won't be the first in a sequence of the same priority
> tasks. But it seems to me it gives nothing.

Looks reasonable, although I can't say I really understand this code.
Add Gregory.

> Signed-off-by: Tkhai Kirill <tkhai@yandex.ru>
>
> --- kernel/sched_rt.c.orig 2011-12-02 00:29:11.970243145 +0400
> +++ kernel/sched_rt.c 2011-12-02 00:37:43.622846606 +0400

please use -p1

> @@ -1572,43 +1572,37 @@ static void set_cpus_allowed_rt(struct t
> const struct cpumask *new_mask)
> {
> int weight = cpumask_weight(new_mask);
> + struct rq *rq;
>
> BUG_ON(!rt_task(p));
>
> /*
> - * Update the migration status of the RQ if we have an RT task
> - * which is running AND changing its weight value.
> + * Just exit if it's not necessary to change migration status
> */
> - if (p->on_rq && (weight != p->rt.nr_cpus_allowed)) {
> - struct rq *rq = task_rq(p);
> + if ((p->rt.nr_cpus_allowed <= 1 && weight <= 1)
> + || (p->rt.nr_cpus_allowed > 1 && weight > 1))
> + return;

Subjective, but may be

if ((p->rt.nr_cpus_allowed > 1) != (weight > 1))
return;

looks more understandable?

> - if (!task_current(rq, p)) {
> - /*
> - * Make sure we dequeue this task from the pushable list
> - * before going further. It will either remain off of
> - * the list because we are no longer pushable, or it
> - * will be requeued.
> - */
> - if (p->rt.nr_cpus_allowed > 1)
> - dequeue_pushable_task(rq, p);
> -
> - /*
> - * Requeue if our weight is changing and still > 1
> - */
> - if (weight > 1)
> - enqueue_pushable_task(rq, p);
> -
> - }
> -
> - if ((p->rt.nr_cpus_allowed <= 1) && (weight > 1)) {
> - rq->rt.rt_nr_migratory++;
> - } else if ((p->rt.nr_cpus_allowed > 1) && (weight <= 1)) {
> - BUG_ON(!rq->rt.rt_nr_migratory);
> - rq->rt.rt_nr_migratory--;
> - }
> + if (!p->on_rq)
> + return;
>
> - update_rt_migration(&rq->rt);
> + rq = task_rq(p);
> +
> + /*
> + * Several cpus were allowed but now it's not so OR vice versa
> + */
> + if (weight <= 1) {
> + if (!task_current(rq, p))
> + dequeue_pushable_task(rq, p);
> + BUG_ON(!rq->rt.rt_nr_migratory);
> + rq->rt.rt_nr_migratory--;
> + } else {
> + if (!task_current(rq, p))
> + enqueue_pushable_task(rq, p);
> + rq->rt.rt_nr_migratory++;
> }
> +
> + update_rt_migration(&rq->rt);
> }
>
> /* Assumes rq->lock is held */
>
>
>