Commit 6efdb105 authored Aug 20, 2015 by Byungchul Park Committed by Ingo Molnar Sep 13, 2015

sched/fair: Fix switched_to_fair()'s per entity load tracking



Where switched_from_fair() will remove the entity's load from the
runqueue, switched_to_fair() does not currently add it back. This
means that when a task leaves the fair class for a short duration; say
because of PI; we loose its load contribution.

This can ripple forward and disturb the load tracking because other
operations (enqueue, dequeue) assume its factored in. Only once the
runqueue empties will the load tracking recover.

When we add it back in, age the per entity average to match up with
the runqueue age. This has the obvious problem that if the task leaves
the fair class for a significant time, the load will age to 0.

Employ the normal migration rule for inter-runqueue moves in
task_move_group_fair(). Again, there is the obvious problem of the
task migrating while not in the fair class.

The alternative solution would be to to omit the chunk in
attach_entity_load_avg(), which would effectively reset the timestamp
and use whatever avg there was.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
[ Rewrote the changelog and comments. ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1440069720-27038-5-git-send-email-byungchul.park@lge.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 1746babb

kernel/sched/fair.c

+23 −0

Original line number	Diff line number	Diff line
		@@ -2712,6 +2712,20 @@ static inline void update_load_avg(struct sched_entity *se, int update_tg)

		static void attach_entity_load_avg(struct cfs_rq cfs_rq, struct sched_entity se)
		{
		/*
		* If we got migrated (either between CPUs or between cgroups) we'll
		* have aged the average right before clearing @last_update_time.
		*/
		if (se->avg.last_update_time) {
		__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
		&se->avg, 0, 0, NULL);

		/*
		* XXX: we could have just aged the entire load away if we've been
		* absent from the fair class for too long.
		*/
		}

		se->avg.last_update_time = cfs_rq->avg.last_update_time;
		cfs_rq->avg.load_avg += se->avg.load_avg;
		cfs_rq->avg.load_sum += se->avg.load_sum;
		@@ -7945,6 +7959,9 @@ static void switched_to_fair(struct rq rq, struct task_struct p)
		se->depth = se->parent ? se->parent->depth + 1 : 0;
		#endif

		/* Synchronize task with its cfs_rq */
		attach_entity_load_avg(cfs_rq_of(&p->se), &p->se);

		if (!task_on_rq_queued(p)) {

		/*
		@@ -8044,6 +8061,12 @@ static void task_move_group_fair(struct task_struct *p, int queued)
		/* Synchronize task with its prev cfs_rq */
		detach_entity_load_avg(cfs_rq, se);
		set_task_rq(p, task_cpu(p));

		#ifdef CONFIG_SMP
		/* Tell se's cfs_rq has been changed -- migrated */
		p->se.avg.last_update_time = 0;
		#endif

		se->depth = se->parent ? se->parent->depth + 1 : 0;
		cfs_rq = cfs_rq_of(se);
		if (!queued)

Admin message