[PATCH 1/1] [PATCH v3]sched/pelt: Fix the attach_entity_load_avg calculate method

[Kuyo Chang]

From: kuyo chang <kuyo.chang@xxxxxxxxxxxx>

I meet the warning message at cfs_rq_is_decayed at below code.

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

Following is the calltrace.

Call trace:
__update_blocked_fair
update_blocked_averages
newidle_balance
pick_next_task_fair
__schedule
schedule
pipe_read
vfs_read
ksys_read

After code analyzing and some debug messages, I found it exits a corner
case at attach_entity_load_avg which will cause load_sum is null but
load_avg is not.
Consider se_weight is 88761 according by sched_prio_to_weight table.
And assume the get_pelt_divider() is 47742, se->avg.load_avg is 1.
By the calculating for se->avg.load_sum as following will become zero
as following.
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.

After enqueue_load_avg code as below.
cfs_rq->avg.load_avg += se->avg.load_avg;
cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;

Then the load_sum for cfs_rq will be 1 while the load_sum for cfs_rq is 0.
So it will hit the warning message.

In order to fix the corner case, make sure the se->load_avg|sum is correct
before enqueue_load_avg.

After long time testing, the kernel warning was gone and the system runs
as well as before.

Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq and its se")
Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
---

 v1->v2:
 (1)Thanks for suggestion from Peter Zijlstra & Vincent Guittot.
 (2)By suggestion from Vincent Guittot,
 rework the se->load_sum calculation method for fix the corner case,
 make sure the se->load_avg|sum is correct before enqueue_load_avg.
 (3)Rework changlog.

 v2->v3:
 (1)Rename Subject. 
 (1)Add fix tag.


 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4bd299d67ab..159274482c4e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3829,10 +3829,12 @@ 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 = se->avg.load_avg * divider;
+	if (se_weight(se) < se->avg.load_sum) {
 		se->avg.load_sum =
-			div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
+			div_u64(se->avg.load_sum, se_weight(se));
+	} else {
+		se->avg.load_sum = 1;
 	}
 
 	enqueue_load_avg(cfs_rq, se);
-- 
2.18.0