linux-kernel - Re: [PATCH]sched_rt.c: Avoid unnecessary dequeue and enqueue of pushable tasks in set_cpus_allowed

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Date:	Sun, 19 Feb 2012 18:17:29 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH]sched_rt.c: Avoid unnecessary dequeue and enqueue of pushable tasks in set_cpus_allowed_rt()

13.02.2012, 21:23, "Steven Rostedt" <rostedt@...dmis.org>:
> I wasn't on the Cc of the original message, but it was bounced to me
> awhile ago. I'm cleaning out my email and came across it.
>
> Can you send me the latest version of this patch, either against latest
> Linus, or against tip/master.
>
> Thanks,
>
> -- Steve
>
> P.S. I'll be at ELC this week so it may not get processed right away.
>

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 (or not pushable). We are not insterested in its exact
values, 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.

Signed-off-by: Tkhai Kirill <tkhai@...dex.ru>
---

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 3640ebb..bf48343 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1779,43 +1779,36 @@ 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 */
--
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