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

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Message-ID: <20251030060143.GE33@bytedance>
Date: Thu, 30 Oct 2025 14:01:43 +0800
From: Aaron Lu <ziqianlu@...edance.com>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: 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>, 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

On Thu, Oct 30, 2025 at 11:16:36AM +0530, K Prateek Nayak wrote:
> 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 Also, update outdated comments in tg_throttle_down() since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>

Thank you Prateek!