linux-kernel - Re: [PATCH 1/4] sched/fair: Propagate load for throttled cfs

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Message-ID: <f13922e4-07f8-4720-bc62-c309efc48967@linux.dev>
Date: Wed, 10 Sep 2025 20:36:08 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Aaron Lu <ziqianlu@...edance.com>,
 Valentin Schneider <vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 Peter Zijlstra <peterz@...radead.org>, Josh Don <joshdon@...gle.com>,
 Ingo Molnar <mingo@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Xi Wang <xii@...gle.com>
Cc: linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>,
 Chuyi Zhou <zhouchuyi@...edance.com>, Jan Kiszka <jan.kiszka@...mens.com>,
 Florian Bezdeka <florian.bezdeka@...mens.com>,
 Songtang Liu <liusongtang@...edance.com>, Chen Yu <yu.c.chen@...el.com>,
 Matteo Martelli <matteo.martelli@...ethink.co.uk>,
 Michal Koutný <mkoutny@...e.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH 1/4] sched/fair: Propagate load for throttled cfs_rq

On 2025/9/10 17:50, Aaron Lu wrote:
> Before task based throttle model, propagating load will stop at a
> throttled cfs_rq and that propagate will happen on unthrottle time by
> update_load_avg().
> 
> Now that there is no update_load_avg() on unthrottle for throttled
> cfs_rq and all load tracking is done by task related operations, let the
> propagate happen immediately.
> 
> While at it, add a comment to explain why cfs_rqs that are not affected
> by throttle have to be added to leaf cfs_rq list in
> propagate_entity_cfs_rq() per my understanding of commit 0258bdfaff5b
> ("sched/fair: Fix unfairness caused by missing load decay").
> 
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>

LGTM!

Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>

Thanks.

> ---
>   kernel/sched/fair.c | 26 ++++++++++++++++++--------
>   1 file changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df8dc389af8e1..f993de30e1466 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5729,6 +5729,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
>   	return cfs_bandwidth_used() && cfs_rq->throttled;
>   }
>   
> +static inline bool cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq)
> +{
> +	return cfs_bandwidth_used() && cfs_rq->pelt_clock_throttled;
> +}
> +
>   /* check whether cfs_rq, or any parent, is throttled */
>   static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
>   {
> @@ -6721,6 +6726,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
>   	return 0;
>   }
>   
> +static inline bool cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq)
> +{
> +	return false;
> +}
> +
>   static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
>   {
>   	return 0;
> @@ -13151,10 +13161,13 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
>   {
>   	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>   
> -	if (cfs_rq_throttled(cfs_rq))
> -		return;
> -
> -	if (!throttled_hierarchy(cfs_rq))
> +	/*
> +	 * If a task gets attached to this cfs_rq and before being queued,
> +	 * it gets migrated to another CPU due to reasons like affinity
> +	 * change, make sure this cfs_rq stays on leaf cfs_rq list to have
> +	 * that removed load decayed or it can cause faireness problem.
> +	 */
> +	if (!cfs_rq_pelt_clock_throttled(cfs_rq))
>   		list_add_leaf_cfs_rq(cfs_rq);
>   
>   	/* Start to propagate at parent */
> @@ -13165,10 +13178,7 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
>   
>   		update_load_avg(cfs_rq, se, UPDATE_TG);
>   
> -		if (cfs_rq_throttled(cfs_rq))
> -			break;
> -
> -		if (!throttled_hierarchy(cfs_rq))
> +		if (!cfs_rq_pelt_clock_throttled(cfs_rq))
>   			list_add_leaf_cfs_rq(cfs_rq);
>   	}
>   }