sched/fair: Cure calc_cfs_shares() vs. reweight_entity()

Vincent reported that when running in a cgroup, his root
cfs_rq->avg.load_avg dropped to 0 on task idle.

This is because reweight_entity() will now immediately propagate the
weight change of the group entity to its cfs_rq, and as it happens,
our approxmation (5) for calc_cfs_shares() results in 0 when the group
is idle.

Avoid this by using the correct (3) as a lower bound on (5). This way
the empty cgroup will slowly decay instead of instantly drop to 0.

Reported-by: Vincent Guittot <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dd565aeafc5a..63166a0ed854 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2763,11 +2763,10 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq)
 	tg_shares = READ_ONCE(tg->shares);
 
 	/*
-	 * This really should be: cfs_rq->avg.load_avg, but instead we use
-	 * cfs_rq->load.weight, which is its upper bound. This helps ramp up
-	 * the shares for small weight interactive tasks.
+	 * Because (5) drops to 0 when the cfs_rq is idle, we need to use (3)
+	 * as a lower bound.
 	 */
-	load = scale_load_down(cfs_rq->load.weight);
+	load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
 
 	tg_weight = atomic_long_read(&tg->load_avg);