linux-kernel - Re: [PATCH v3] sched/fair: Fix unfairness caused by stalled tg_load_avg

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Message-ID: <20250826081904.GB87@bytedance>
Date: Tue, 26 Aug 2025 16:19:04 +0800
From: Aaron Lu <ziqianlu@...edance.com>
To: xupengbo <xupengbo@...o.com>
Cc: 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>,
	David Vernet <void@...ifault.com>, Aaron Lu <aaron.lu@...el.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	xupengbo1029@....com
Subject: Re: [PATCH v3] sched/fair: Fix unfairness caused by stalled
 tg_load_avg_contrib when the last task migrates out.

On Tue, Aug 26, 2025 at 03:57:42PM +0800, xupengbo wrote:
> 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@...o.com>

With the below typo fixed:

Tested-by: Aaron Lu <ziqianlu@...edance.com>
Reviewed-by: Aaron Lu <ziqianlu@...edance.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@....com>
> after September 3, 2025, after that date <xupengbo@...o.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)
                       ~~~~
typo: tg_load_avg_contrib

> +		return false;
> +
>  	return true;
>  }
>  
> 
> base-commit: fab1beda7597fac1cecc01707d55eadb6bbe773c
> -- 
> 2.43.0
>