[PATCH 5.15 089/124] sched/pelt: Fix attach_entity_load_avg() corner case

Greg Kroah-Hartman
2022-04-26T09:09:27Z

From: kuyo chang <kuyo.chang@mediatek.com>

[ Upstream commit 40f5aa4c5eaebfeaca4566217cb9c468e28ed682 ]

The warning in cfs_rq_is_decayed() triggered:

SCHED_WARN_ON(cfs_rq->avg.load_avg ||
cfs_rq->avg.util_avg ||
cfs_rq->avg.runnable_avg)

There exists a corner case in attach_entity_load_avg() which will
cause load_sum to be zero while load_avg will not be.

Consider se_weight is 88761 as per the sched_prio_to_weight[] table.
Further assume the get_pelt_divider() is 47742, this gives:
se->avg.load_avg is 1.

However, calculating load_sum:

se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
se->avg.load_sum = 1*47742/88761 = 0.

Then enqueue_load_avg() adds this to the cfs_rq totals:

cfs_rq->avg.load_avg += se->avg.load_avg;
cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;

Resulting in load_avg being 1 with load_sum is 0, which will trigger
the WARN.

Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq and its se")
Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
[peterz: massage changelog]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lkml.kernel.org/r/20220414090229.342-1-kuyo.chang@mediatek.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02766f3fe206..9a4fa22a69ed 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3794,11 +3794,11 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s

se->avg.runnable_sum = se->avg.runnable_avg * divider;

- se->avg.load_sum = divider;
- if (se_weight(se)) {
- se->avg.load_sum =
- div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
- }
+ se->avg.load_sum = se->avg.load_avg * divider;
+ if (se_weight(se) < se->avg.load_sum)
+ se->avg.load_sum = div_u64(se->avg.load_sum, se_weight(se));
+ else
+ se->avg.load_sum = 1;

enqueue_load_avg(cfs_rq, se);
cfs_rq->avg.util_avg += se->avg.util_avg;
--
2.35.1