sched/pelt: Fix attach_entity_load_avg() corner case

stable inclusion
from stable-v5.10.113
commit 88fcfd6ee6c5a617e712b346e9c15fc3057e532e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5ISAH

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88fcfd6ee6c5a617e712b346e9c15fc3057e532e

--------------------------------

[ Upstream commit 40f5aa4c ]

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: f207934f ("sched/fair: Align PELT windows between cfs_rq and its se")
Signed-off-by: Nkuyo chang <kuyo.chang@mediatek.com>
[peterz: massage changelog]
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NVincent Guittot <vincent.guittot@linaro.org>
Tested-by: NDietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lkml.kernel.org/r/20220414090229.342-1-kuyo.chang@mediatek.comSigned-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

kernel/sched/fair.c

@@ -3771,11 +3771,11 @@ 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;