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

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Message-ID: <20251014090728.GA41@bytedance>
Date: Tue, 14 Oct 2025 17:11:13 +0800
From: Aaron Lu <ziqianlu@...edance.com>
To: Hao Jia <jiahao.kernel@...il.com>
Cc: Valentin Schneider <vschneid@...hat.com>,
	Ben Segall <bsegall@...gle.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	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] sched/fair: Prevent cfs_rq from being unthrottled with
 zero runtime_remaining

Hi Hao,

On Tue, Oct 14, 2025 at 03:43:10PM +0800, Hao Jia wrote:
> 
> Hello Aaron,
> 
> On 2025/9/29 15:46, Aaron Lu wrote:
> > When a cfs_rq is to be throttled, its limbo list should be empty and
> > that's why there is a warn in tg_throttle_down() for non empty
> > cfs_rq->throttled_limbo_list.
> > 
> > When running a test with the following hierarchy:
> > 
> >            root
> >          /      \
> >          A*     ...
> >       /  |  \   ...
> >          B
> >         /  \
> >        C*
> > 
> > where both A and C have quota settings, that warn on non empty limbo list
> > is triggered for a cfs_rq of C, let's call it cfs_rq_c(and ignore the cpu
> > part of the cfs_rq for the sake of simpler representation).
> > 
> 
> I encountered a similar warning a while ago and fixed it. I have a question
> I'd like to ask. tg_unthrottle_up(cfs_rq_C) calls enqueue_task_fair(p) to
> enqueue a task, which requires that the runtime_remaining of task p's entire
> task_group hierarchy be greater than 0.
> 
> In addition to the case you fixed above,
> When bandwidth is running normally, Is it possible that there's a corner
> case where cfs_A->runtime_remaining > 0, but cfs_B->runtime_remaining < 0
> could trigger a similar warning?

Do you mean B also has quota set and cfs_B's runtime_remaining < 0?
In this case, B should be throttled and C is a descendent of B so should
also be throttled, i.e. C can't be unthrottled when B is in throttled
state. Do I understand you correctly?

> 
> So, I previously tried to fix this issue using the following code, adding
> the ENQUEUE_THROTTLE flag to ensure that tasks enqueued in
> tg_unthrottle_up() aren't throttled.
> 

Yeah I think this can also fix the warning.
I'm not sure if it is a good idea though, because on unthrottle, the
expectation is, this cfs_rq should have runtime_remaining > 0 and if
it's not the case, I think it is better to know why.

Thanks.