Commit 20bbd926 authored Oct 19, 2016 by Vincent Guittot Committed by Andres Oportus Jun 02, 2017

UPSTREAM: sched/fair: Fix incorrect task group ->load_avg



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.

Change-Id: Ie55021ff98ba49016adfddb2444e9c9709939226
Reported-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@vger.kernel.org> # 4.8.x
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: joonwoop@codeaurora.org
Fixes: 3d30544f0212 ("sched/fair: Apply more PELT fixes)
Link: http://lkml.kernel.org/r/1476881123-10159-1-git-send-email-vincent.guittot@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit b5a9b340789b2b24c6896bcf7a065c31a4db671c)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

parent 640c909c

kernel/sched/fair.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -739,6 +739,13 @@ void init_entity_runnable_average(struct sched_entity *se)
		* will definitely be update (after enqueue).
		*/
		sa->period_contrib = 1023;
		/*
		* 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;
		/*