linux-kernel - Re: [RFC PATCH 7/7] sched/fair: Make sure cfs_rq has enough runtime

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Message-ID: <947ab5ab-3fbe-4a5c-bff5-6c257c7c5ba1@amd.com>
Date: Fri, 14 Mar 2025 09:48:00 +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 7/7] sched/fair: Make sure cfs_rq has enough
 runtime_remaining on unthrottle path

Hello Aaron,

On 3/13/2025 12:52 PM, Aaron Lu wrote:
> It's possible unthrottle_cfs_rq() is called with !runtime_remaining
> due to things like user changed quota setting(see tg_set_cfs_bandwidth())
> or async unthrottled us with a positive runtime_remaining but other still
> running entities consumed those runtime before we reach there.
> 
> Anyway, we can't unthrottle this cfs_rq without any runtime remaining
> because task enqueue during unthrottle can immediately trigger a throttle
> by check_enqueue_throttle(), which should never happen.
> 
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
> ---
>   kernel/sched/fair.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index be96f7d32998c..d646451d617c1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6058,6 +6058,19 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>   	struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
>   	struct sched_entity *se = cfs_rq->tg->se[cpu_of(rq)];
> 
> +	/*
> +	 * It's possible we are called with !runtime_remaining due to things
> +	 * like user changed quota setting(see tg_set_cfs_bandwidth()) or async
> +	 * unthrottled us with a positive runtime_remaining but other still
> +	 * running entities consumed those runtime before we reach here.
> +	 *
> +	 * Anyway, we can't unthrottle this cfs_rq without any runtime remaining
> +	 * because any enqueue below will immediately trigger a throttle, which
> +	 * is not supposed to happen on unthrottle path.
> +	 */
> +	if (cfs_rq->runtime_enabled && !cfs_rq->runtime_remaining)

Should this be "cfs_rq->runtime_remaining <= 0" since slack could have
built up by that time we come here?

> +		return;
> +
>   	cfs_rq->throttled = 0;
> 
>   	update_rq_clock(rq);

-- 
Thanks and Regards,
Prateek