[tip: sched/urgent] sched/fair: Make sure to update tg contrib for blocked load

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     02da26ad5ed6ea8680e5d01f20661439611ed776
Gitweb:        https://git.kernel.org/tip/02da26ad5ed6ea8680e5d01f20661439611ed776
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Thu, 27 May 2021 14:29:16 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 31 May 2021 10:14:48 +02:00

sched/fair: Make sure to update tg contrib for blocked load

During the update of fair blocked load (__update_blocked_fair()), we
update the contribution of the cfs in tg->load_avg if cfs_rq's pelt
has decayed.  Nevertheless, the pelt values of a cfs_rq could have
been recently updated while propagating the change of a child. In this
case, cfs_rq's pelt will not decayed because it has already been
updated and we don't update tg->load_avg.

__update_blocked_fair
  ...
  for_each_leaf_cfs_rq_safe: child cfs_rq
    update cfs_rq_load_avg() for child cfs_rq
    ...
    update_load_avg(cfs_rq_of(se), se, 0)
      ...
      update cfs_rq_load_avg() for parent cfs_rq
		-propagation of child's load makes parent cfs_rq->load_sum
		 becoming null
        -UPDATE_TG is not set so it doesn't update parent
		 cfs_rq->tg_load_avg_contrib
  ..
  for_each_leaf_cfs_rq_safe: parent cfs_rq
    update cfs_rq_load_avg() for parent cfs_rq
      - nothing to do because parent cfs_rq has already been updated
		recently so cfs_rq->tg_load_avg_contrib is not updated
    ...
    parent cfs_rq is decayed
      list_del_leaf_cfs_rq parent cfs_rq
	  - but it still contibutes to tg->load_avg

we must set UPDATE_TG flags when propagting pending load to the parent

Fixes: 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
Reported-by: Odin Ugedal <odin@xxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Odin Ugedal <odin@xxxxxxx>
Link: https://lkml.kernel.org/r/20210527122916.27683-3-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f4795b8..e7c8277 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8029,7 +8029,7 @@ static bool __update_blocked_fair(struct rq *rq, bool *done)
 		/* Propagate pending load changes to the parent, if any: */
 		se = cfs_rq->tg->se[cpu];
 		if (se && !skip_blocked_update(se))
-			update_load_avg(cfs_rq_of(se), se, 0);
+			update_load_avg(cfs_rq_of(se), se, UPDATE_TG);
 
 		/*
 		 * There can be a lot of idle CPU cgroups.  Don't let fully