[PATCH v3] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

[xupengbo <xupengbo@oppo.com>]

When a task is migrated out, there is a probability that the tg->load_avg
value will become abnormal. The reason is as follows.

1. Due to the 1ms update period limitation in update_tg_load_avg(), there
is a possibility that the reduced load_avg is not updated to tg->load_avg
when a task migrates out.
2. Even though __update_blocked_fair() traverses the leaf_cfs_rq_list and
calls update_tg_load_avg() for cfs_rqs that are not fully decayed, the key
function cfs_rq_is_decayed() does not check whether
cfs->tg_load_avg_contrib is null. Consequently, in some cases,
__update_blocked_fair() removes cfs_rqs whose avg.load_avg has not been
updated to tg->load_avg.

I added a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
which blocks the case (2.) mentioned above.
After some preliminary discussion and analysis, I think it is feasible to
directly check if cfs_rq->tg_load_avg_contrib is 0 in cfs_rq_is_decay().
So patch v3 was submitted.

Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
Signed-off-by: xupengbo <xupengbo@oppo.com>
---
Changes:
v1 -> v2: 
- Another option to fix the bug. Check cfs_rq->tg_load_avg_contrib in 
cfs_rq_is_decayed() to avoid early removal from the leaf_cfs_rq_list.
- Link to v1 : https://lore.kernel.org/cgroups/20250804130326.57523-1-xupengbo@oppo.com/
v2 -> v3:
- Check if cfs_rq->tg_load_avg_contrib is 0 derectly.
- Link to v2 : https://lore.kernel.org/cgroups/20250805144121.14871-1-xupengbo@oppo.com/

Please send emails to a different email address <xupengbo1029@163.com>
after September 3, 2025, after that date <xupengbo@oppo.com> will expire
for personal reasons.

Thanks,
Xu Pengbo

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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b173a059315c..df348cb6a5f3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4062,6 +4062,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	if (child_cfs_rq_on_list(cfs_rq))
 		return false;
 
+	if (cfs_rq->tg_laod_avg_contrib)
+		return false;
+
 	return true;
 }
 

base-commit: fab1beda7597fac1cecc01707d55eadb6bbe773c
-- 
2.43.0