linux-kernel - [PATCH v2] sched/fair: Fix unfairness caused by stalled tg_load_avg

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Message-ID: <20250805144121.14871-1-xupengbo@oppo.com>
Date: Tue, 5 Aug 2025 22:41:20 +0800
From: xupengbo <xupengbo@...o.com>
To: <ziqianlu@...edance.com>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra
	<peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, "Mel
 Gorman" <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Aaron Lu
	<aaron.lu@...el.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	<linux-kernel@...r.kernel.org>
CC: <xupengbo@...o.com>, <cgroups@...r.kernel.org>
Subject: [PATCH v2] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

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. I follow the condition in
update_tg_load_avg() instead of directly checking if
cfs_rq->tg_load_avg_contrib is null. I think it's necessary to keep the
condition consistent in both places, otherwise unexpected problems may
occur.

Thanks for your comments,
Xu Pengbo

Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
Signed-off-by: xupengbo <xupengbo@...o.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/T/#u

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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b173a059315c..a35083a2d006 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4062,6 +4062,11 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	if (child_cfs_rq_on_list(cfs_rq))
 		return false;

+	long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
+
+	if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64)
+		return false;
+
 	return true;
 }

-- 
2.43.0