linux-kernel - Re: [RFC PATCH 4/7] sched/fair: Take care of migrated task for task based throttle

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Message-ID: <f091f0e8-ead5-4fed-afbf-45be29db8e2e@amd.com>
Date: Fri, 14 Mar 2025 09:33:10 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>, Valentin Schneider
	<vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>, Peter Zijlstra
	<peterz@...radead.org>, Josh Don <joshdon@...gle.com>, Ingo Molnar
	<mingo@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
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>, Chengming Zhou
	<chengming.zhou@...ux.dev>, Chuyi Zhou <zhouchuyi@...edance.com>
Subject: Re: [RFC PATCH 4/7] sched/fair: Take care of migrated task for task
 based throttle

Hello Aaron,

On 3/13/2025 12:51 PM, Aaron Lu wrote:
> If a task is migrated to a new cpu, it is possible this task is not
> throttled but the new cfs_rq is throttled or vice vesa. Take care of
> these situations in enqueue path.
> 
> Note that we can't handle this in migrate_task_rq_fair() because there,
> the dst cpu's rq lock is not held and things like checking if the new
> cfs_rq needs throttle can be racy.
> 
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
> ---
>   kernel/sched/fair.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4a95fe3785e43..9e036f18d73e6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7051,6 +7051,23 @@ enqueue_task_fair(struct rq *rq, struct
> task_struct *p, int flags)
>   	assert_list_leaf_cfs_rq(rq);
> 
>   	hrtick_update(rq);
> +
> +	if (!cfs_bandwidth_used())
> +		return;
> +
> +	/*
> +	 * This is for migrate_task_rq_fair(): the new_cpu's rq lock is not held
> +	 * in migrate_task_rq_fair() so we have to do these things in enqueue
> +	 * time when the dst cpu's rq lock is held. Doing this check in enqueue
> +	 * time also takes care of newly woken up tasks, e.g. a task wakes up
> +	 * into a throttled cfs_rq.
> +	 *
> +	 * It's possible the task has a throttle work added but this new cfs_rq
> +	 * is not in throttled hierarchy but that's OK, throttle_cfs_rq_work()
> +	 * will take care of it.
> +	 */
> +	if (throttled_hierarchy(cfs_rq_of(&p->se)))
> +		task_throttle_setup_work(p);

Any reason we can't move this to somewhere towards the top?
throttled_hierarchy() check should be cheap enough and we probably don't
need the cfs_bandwidth_used() guarding check unless there are other
concerns that I may have missed.

>   }
> 
>   static void set_next_buddy(struct sched_entity *se);

-- 
Thanks and Regards,
Prateek