[PATCH 4.8 108/140] sched/fair: Fix incorrect task group ->load_avg

[Greg Kroah-Hartman]

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

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.

Reported-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx>
Tested-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: joonwoop@xxxxxxxxxxxxxx
Fixes: 3d30544f0212 ("sched/fair: Apply more PELT fixes)
Link: http://lkml.kernel.org/r/1476881123-10159-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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