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

[Oleg Nesterov <oleg@redhat.com>]

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 */
> 
> 
>