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authorVincent Guittot <vincent.guittot@linaro.org>2016-10-19 14:45:23 +0200
committerIngo Molnar <mingo@kernel.org>2016-10-19 15:04:47 +0200
commitb5a9b340789b2b24c6896bcf7a065c31a4db671c (patch)
tree25985e88c736bf449970677e8eddae6ea75c9045 /kernel/sched
parent1a1891d762d6e64daf07b5be4817e3fbb29e3c59 (diff)
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. 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>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/fair.c9
1 files changed, 8 insertions, 1 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 76ee7de1859d..d941c97dfbc3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -690,7 +690,14 @@ void init_entity_runnable_average(struct sched_entity *se)
* 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