linux-kernel - Re: [PATCH v4 3/5] sched/fair: Switch to task based throttle model

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Message-ID: <20250905125328.GU3245006@noisy.programming.kicks-ass.net>
Date: Fri, 5 Sep 2025 14:53:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Aaron Lu <ziqianlu@...edance.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Ben Segall <bsegall@...gle.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 Koutn?? <mkoutny@...e.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v4 3/5] sched/fair: Switch to task based throttle model

On Fri, Sep 05, 2025 at 07:37:19PM +0800, Aaron Lu wrote:
> Hi Peter,
> 
> On Thu, Sep 04, 2025 at 03:04:07PM +0800, Aaron Lu wrote:
> > On Thu, Sep 04, 2025 at 11:14:31AM +0530, K Prateek Nayak wrote:
> > > On 9/4/2025 1:57 AM, Peter Zijlstra wrote:
> > > > So this is mostly tasks leaving/joining the class/cgroup. And its
> > > > purpose seems to be to remove/add the blocked load component.
> > > > 
> > > > Previously throttle/unthrottle would {de,en}queue the whole subtree from
> > > > PELT, see how {en,de}queue would also stop at throttle.
> > > > 
> > > > But now none of that is done; PELT is fully managed by the tasks
> > > > {de,en}queueing.
> > > > 
> > > > So I'm thinking that when a task joins fair (deboost from RT or
> > > > whatever), we add the blocking load and fully propagate it. If the task
> > > > is subject to throttling, that will then happen 'naturally' and it will
> > > > dequeue itself again.
> > > 
> > > That seems like the correct thing to do yes. Those throttled_cfs_rq()
> > > checks in propagate_entity_cfs_rq() can be removed then.
> > >
> > 
> > Not sure if I understand correctly, I've come to the below code
> > according to your discussion:
> >
> 
> Does the below diff look sane to you? If so, shall I send a separate
> patch on top or fold it in patch3 and send an updated patch3?

Yeah, I suppose that works. Please send a follow up patch. It would also
be good to have a comment that explains why we need that list_add_leaf
thing. I think I see, but I'm sure I'll have forgotten all next time I
see this code.