releases/4.8.5/sched-fair-fix-incorrect-task-group-load_avg.patch

   1 From b5a9b340789b2b24c6896bcf7a065c31a4db671c Mon Sep 17 00:00:00 2001
   2 From: Vincent Guittot <vincent.guittot@linaro.org>
   3 Date: Wed, 19 Oct 2016 14:45:23 +0200
   4 Subject: sched/fair: Fix incorrect task group ->load_avg
   5
   6 From: Vincent Guittot <vincent.guittot@linaro.org>
   7
   8 commit b5a9b340789b2b24c6896bcf7a065c31a4db671c upstream.
   9
  10 A scheduler performance regression has been reported by Joseph Salisbury,
  11 which he bisected back to:
  12
  13   3d30544f0212 ("sched/fair: Apply more PELT fixes)
  14
  15 The regression triggers when several levels of task groups are involved
  16 (read: SystemD) and cpu_possible_mask != cpu_present_mask.
  17
  18 The root cause is that group entity's load (tg_child->se[i]->avg.load_avg)
  19 is initialized to scale_load_down(se->load.weight). During the creation of
  20 a child task group, its group entities on possible CPUs are attached to
  21 parent's cfs_rq (tg_parent) and their loads are added to the parent's load
  22 (tg_parent->load_avg) with update_tg_load_avg().
  23
  24 But only the load on online CPUs will then be updated to reflect real load,
  25 whereas load on other CPUs will stay at the initial value.
  26
  27 The result is a tg_parent->load_avg that is higher than the real load, the
  28 weight of group entities (tg_parent->se[i]->load.weight) on online CPUs is
  29 smaller than it should be, and the task group gets a less running time than
  30 what it could expect.
  31
  32 ( This situation can be detected with /proc/sched_debug. The ".tg_load_avg"
  33   of the task group will be much higher than sum of ".tg_load_avg_contrib"
  34   of online cfs_rqs of the task group. )
  35
  36 The load of group entities don't have to be intialized to something else
  37 than 0 because their load will increase when an entity is attached.
  38
  39 Reported-by: Joseph Salisbury <joseph.salisbury@canonical.com>
  40 Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
  41 Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
  42 Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
  43 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  44 Cc: Peter Zijlstra <peterz@infradead.org>
  45 Cc: Thomas Gleixner <tglx@linutronix.de>
  46 Cc: joonwoop@codeaurora.org
  47 Fixes: 3d30544f0212 ("sched/fair: Apply more PELT fixes)
  48 Link: http://lkml.kernel.org/r/1476881123-10159-1-git-send-email-vincent.guittot@linaro.org
  49 Signed-off-by: Ingo Molnar <mingo@kernel.org>
  50 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  51
  52 ---
  53  kernel/sched/fair.c |    9 ++++++++-
  54  1 file changed, 8 insertions(+), 1 deletion(-)
  55
  56 --- a/kernel/sched/fair.c
  57 +++ b/kernel/sched/fair.c
  58 @@ -680,7 +680,14 @@ void init_entity_runnable_average(struct
  59          * will definitely be update (after enqueue).
  60          */
  61         sa->period_contrib = 1023;
  62 -       sa->load_avg = scale_load_down(se->load.weight);
  63 +       /*
  64 +        * Tasks are intialized with full load to be seen as heavy tasks until
  65 +        * they get a chance to stabilize to their real load level.
  66 +        * Group entities are intialized with zero load to reflect the fact that
  67 +        * nothing has been attached to the task group yet.
  68 +        */
  69 +       if (entity_is_task(se))
  70 +               sa->load_avg = scale_load_down(se->load.weight);
  71         sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
  72         /*
  73          * At this point, util_avg won't be used in select_task_rq_fair anyway