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

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Message-ID: <20250825095056.GA87@bytedance>
Date: Mon, 25 Aug 2025 17:50:56 +0800
From: Aaron Lu <ziqianlu@...edance.com>
To: xupengbo <xupengbo@...o.com>
Cc: bsegall@...gle.com, cgroups@...r.kernel.org, dietmar.eggemann@....com,
	juri.lelli@...hat.com, linux-kernel@...r.kernel.org,
	mgorman@...e.de, mingo@...hat.com, peterz@...radead.org,
	rostedt@...dmis.org, vincent.guittot@...aro.org,
	vschneid@...hat.com
Subject: Re: [PATCH v2] sched/fair: Fix unfairness caused by stalled
 tg_load_avg_contrib when the last task migrates out.

Hi xupengbo,

On Wed, Aug 06, 2025 at 04:38:10PM +0800, xupengbo wrote:
... ... 
> 
> It actually becomes:
>     if (cfs_rq->tg_load_avg_contrib > 0)
> if cfs_rq->tg_load_avg_contrib == 0 , it will be false. As it is an unsigned
> long, this condition is equivalent to :
>     if (cfs_rq->tg_load_avg_contrib)

I suppose we have reached a conclusion that the right fix is to add a
check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed()? Something
like below:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index af33d107d8034..3ebcb683063f0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4056,6 +4056,9 @@ 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_load_avg_contrib)
+		return false;
+
 	return true;
 }
 
If you also agree, can you send an updated patch to fix this problem?
Thanks.