linux-kernel - Re: [RFC PATCH 00/22] sched/fair: Defer CFS throttling to exit to user mode

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Message-ID: <CABk29NsCqkOVXHfu1=ir9nhKiy2PVsONdZm29qXdbJX8LrfCNA@mail.gmail.com>
Date: Thu, 20 Feb 2025 17:47:00 -0800
From: Josh Don <joshdon@...gle.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, K Prateek Nayak <kprateek.nayak@....com>, 
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Ben Segall <bsegall@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Mel Gorman <mgorman@...e.de>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Clark Williams <clrkwllms@...nel.org>, linux-rt-devel@...ts.linux.dev, 
	Tejun Heo <tj@...nel.org>, Frederic Weisbecker <frederic@...nel.org>, Barret Rhoden <brho@...gle.com>, 
	Petr Mladek <pmladek@...e.com>, Qais Yousef <qyousef@...alina.io>, 
	"Paul E. McKenney" <paulmck@...nel.org>, David Vernet <dvernet@...a.com>, 
	"Gautham R. Shenoy" <gautham.shenoy@....com>, Swapnil Sapkal <swapnil.sapkal@....com>
Subject: Re: [RFC PATCH 00/22] sched/fair: Defer CFS throttling to exit to
 user mode

On Thu, Feb 20, 2025 at 7:40 AM Valentin Schneider <vschneid@...hat.com> wrote:
...
> As pointed by Ben in [1], the issue with the per-task approach is the
> scalability of the unthrottle. You have the rq lock held and you
> potentially end up unthrottling a deep cgroup hierarchy, putting each
> individual task back on its cfs_rq.
>
> I can't find my notes on that in a hurry, but my idea with that for a next
> version was to periodically release the rq lock as we go up the cgroup
> hierarchy during unthrottle - the idea being that we can mess with part of
> hierarchy, and as long as that part isn't connected to the rest (i.e. it's
> not enqueued, like we currently do for CFS throttling), "it should be
> safe".

Can you elaborate a bit more? Even if we periodically release the
lock, we're still spending quite a long time in non-preemptible kernel
context, and unthrottle is also driven by an hrtimer. So we can still
do quite a lot of damage depending on how long the whole loop takes.