linux-kernel - [PATCH 7/7 v4] sched: fix wrong utilization accounting when switching to fair class

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Date:   Mon, 26 Sep 2016 14:19:53 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        kernellwp@...il.com
Cc:     yuyang.du@...el.com, Morten.Rasmussen@....com,
        linaro-kernel@...ts.linaro.org, pjt@...gle.com, bsegall@...gle.com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 7/7 v4] sched: fix wrong utilization accounting when switching to fair class

When a task switches to fair scheduling class, the period between now and
the last update of its utilization is accounted as running time whatever
happened during this period. This wrong accounting applies to the task
and also to the task group branch.

When changing the property of a running task like its list of allowed CPUs
or its scheduling class, we follow the sequence:
-dequeue task
-put task
-change the property
-set task as current task
-enqueue task

The end of the sequence doesn't follow the normal sequence which is :
-enqueue a task
-then set the task as current task.

This wrong ordering is the root cause of wrong utilization accounting.
Update the sequence to follow the right one:
-dequeue task
-put task
-change the property
-enqueue task
-set task as current task

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/core.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3e52d08..7a9c9b9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1105,10 +1105,10 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 
 	p->sched_class->set_cpus_allowed(p, new_mask);
 
-	if (running)
-		p->sched_class->set_curr_task(rq);
 	if (queued)
 		enqueue_task(rq, p, ENQUEUE_RESTORE);
+	if (running)
+		p->sched_class->set_curr_task(rq);
 }
 
 /*
@@ -3687,10 +3687,10 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 
 	p->prio = prio;
 
-	if (running)
-		p->sched_class->set_curr_task(rq);
 	if (queued)
 		enqueue_task(rq, p, queue_flag);
+	if (running)
+		p->sched_class->set_curr_task(rq);
 
 	check_class_changed(rq, p, prev_class, oldprio);
 out_unlock:
@@ -4243,8 +4243,6 @@ static int __sched_setscheduler(struct task_struct *p,
 	prev_class = p->sched_class;
 	__setscheduler(rq, p, attr, pi);
 
-	if (running)
-		p->sched_class->set_curr_task(rq);
 	if (queued) {
 		/*
 		 * We enqueue to tail when the priority of a task is
@@ -4255,6 +4253,8 @@ static int __sched_setscheduler(struct task_struct *p,
 
 		enqueue_task(rq, p, queue_flags);
 	}
+	if (running)
+		p->sched_class->set_curr_task(rq);
 
 	check_class_changed(rq, p, prev_class, oldprio);
 	preempt_disable(); /* avoid rq from going away on us */
@@ -5417,10 +5417,10 @@ void sched_setnuma(struct task_struct *p, int nid)
 
 	p->numa_preferred_nid = nid;
 
-	if (running)
-		p->sched_class->set_curr_task(rq);
 	if (queued)
 		enqueue_task(rq, p, ENQUEUE_RESTORE);
+	if (running)
+		p->sched_class->set_curr_task(rq);
 	task_rq_unlock(rq, p, &rf);
 }
 #endif /* CONFIG_NUMA_BALANCING */
@@ -7868,10 +7868,10 @@ void sched_move_task(struct task_struct *tsk)
 
 	sched_change_group(tsk, TASK_MOVE_GROUP);
 
-	if (unlikely(running))
-		tsk->sched_class->set_curr_task(rq);
 	if (queued)
 		enqueue_task(rq, tsk, ENQUEUE_RESTORE | ENQUEUE_MOVE);
+	if (unlikely(running))
+		tsk->sched_class->set_curr_task(rq);
 
 	task_rq_unlock(rq, tsk, &rf);
 }
-- 
1.9.1