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

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Message-ID: <xhsmh1pvl20ev.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 25 Feb 2025 22:13:12 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Josh Don <joshdon@...gle.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 20/02/25 17:47, Josh Don wrote:
> 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.

Indeed, this only gives the rq lock a breather, but it doesn't help with
preempt / irq off.