Patch "sched/fair: Fix incorrect task group ->load_avg" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

    sched/fair: Fix incorrect task group ->load_avg

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-fair-fix-incorrect-task-group-load_avg.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vincent Guittot <[email protected]>
Date: Wed, 19 Oct 2016 14:45:23 +0200
Subject: sched/fair: Fix incorrect task group ->load_avg

From: Vincent Guittot <[email protected]>

commit b5a9b340789b2b24c6896bcf7a065c31a4db671c upstream.

A scheduler performance regression has been reported by Joseph Salisbury, which he bisected back to:

3d30544f0212 ("sched/fair: Apply more PELT fixes)

The regression triggers when several levels of task groups are involved (read: SystemD) and cpu_possible_mask != cpu_present_mask.

The root cause is that group entity's load (tg_child->se[i]->avg.load_avg) is initialized to scale_load_down(se->load.weight). During the creation of a child task group, its group entities on possible CPUs are attached to parent's cfs_rq (tg_parent) and their loads are added to the parent's load (tg_parent->load_avg) with update_tg_load_avg().

But only the load on online CPUs will then be updated to reflect real load, whereas load on other CPUs will stay at the initial value.

The result is a tg_parent->load_avg that is higher than the real load, the weight of group entities (tg_parent->se[i]->load.weight) on online CPUs is smaller than it should be, and the task group gets a less running time than what it could expect.

( This situation can be detected with /proc/sched_debug. The ".tg_load_avg"
of the task group will be much higher than sum of ".tg_load_avg_contrib" of online cfs_rqs of the task group. )

The load of group entities don't have to be intialized to something else than 0 because their load will increase when an entity is attached.

---
 kernel/sched/fair.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

@@ -680,7 +680,14 @@ void init_entity_runnable_average(struct
 	 * will definitely be update (after enqueue).
 	 */
 	sa->period_contrib = 1023;
-	sa->load_avg = scale_load_down(se->load.weight);
+	/*
+	 * Tasks are intialized with full load to be seen as heavy tasks until
+	 * they get a chance to stabilize to their real load level.
+	 * Group entities are intialized with zero load to reflect the fact that
+	 * nothing has been attached to the task group yet.
+	 */
+	if (entity_is_task(se))
+		sa->load_avg = scale_load_down(se->load.weight);
 	sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
 	/*
 	 * At this point, util_avg won't be used in select_task_rq_fair anyway

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/sched-fair-fix-incorrect-task-group-load_avg.patch