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

[Tkhai Kirill <tkhai@yandex.ru>]

20.12.2011, 21:44, "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
>

Sorry, this time I'm sending "git diffed" output.

>>  @@ -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?

Yes, thanks.

---

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 3640ebb..4467f4d 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1774,43 +1774,36 @@ static void set_cpus_allowed_rt(struct task_struct *p,
 				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 (!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))
+		return;
 
-		}
+	if (!p->on_rq)
+		return;
 
-		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--;
-		}
+	rq = task_rq(p);
 
-		update_rt_migration(&rq->rt);
+	/*
+	 * 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 */