linux-kernel - Re: [PATCH v3] sched/fair: Prevent cfs_rq from being unthrottled with zero runtime

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Message-ID: <0d1f5a59-82b5-494f-b3d3-6332027d0350@amd.com>
Date: Thu, 30 Oct 2025 11:16:36 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>, Ben Segall <bsegall@...gle.com>, Peter
 Zijlstra <peterz@...radead.org>, Hao Jia <jiahao.kernel@...il.com>, Valentin
 Schneider <vschneid@...hat.com>, Chengming Zhou <chengming.zhou@...ux.dev>,
	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 v3] sched/fair: Prevent cfs_rq from being unthrottled with
 zero runtime_remaining

Hello Aaron,

On 10/30/2025 8:57 AM, Aaron Lu wrote:
> I think it's a chaos to trigger throttle on unthrottle path, the status
> of a being unthrottled cfs_rq can be in a mixed state in the end, so fix
> this by granting 1ns to cfs_rq in tg_set_cfs_bandwidth(). This ensures
> cfs_rq_c has a positive runtime_remaining when initialized as unthrottled
> and cannot enter tg_unthrottle_up() with zero runtime_remaining.
> 
> Also, update outdated comments in tg_throttle_down() since
> unthrottle_cfs_rq() is no longer called with zero runtime_remaining.
> While at it, remove a redundant assignment to se in tg_throttle_down().
> 
> Fixes: e1fad12dcb66 ("sched/fair: Switch to task based throttle model")
> Suggested-by: Benjamin Segall <bsegall@...gle.com>
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>

Have been stress testing this on my system and I haven't seen
any splats yet. Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
Tested-by: K Prateek Nayak <kprateek.nayak@....com>

-- 
Thanks and Regards,
Prateek