linux-kernel - Re: [PATCH] sched/cgroup: Lock optimize for cgroup cpu throttle

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Message-ID: <xhsmhzfbvh7nv.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 19 Aug 2025 13:28:52 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Xin Zhao <jackzxcui1989@....com>, tj@...nel.org, hannes@...xchg.org,
 mkoutny@...e.com, mingo@...hat.com, peterz@...radead.org,
 juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, will@...nel.org, boqun.feng@...il.com,
 longman@...hat.com, bigeasy@...utronix.de, clrkwllms@...nel.org
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] sched/cgroup: Lock optimize for cgroup cpu throttle

On 12/08/25 00:04, Xin Zhao wrote:
> On Mon, 2025-08-11 at 22:18 +0800, Sebastian wrote:
>
>> Yeah, please have a look at:
>> https://lore.kernel.org/lkml/20250715071658.267-1-ziqianlu@bytedance.com/
>
>
> Dear Valentin,
>
> In addition to the information in my previous response to Sebastian, I would
> like to add the following point as a reason for my self-recommendation (to
> explore my patch for solving the cgroup performance issue in RT-Linux):
> RT-Linux is a system that places a high emphasis on real-time performance.
> The fact that regular tasks are also included in cgroup groups and throttled
> suggests that they are relatively low-priority tasks that are not expected to
> interfere with high-priority tasks. Therefore, is it not a bit too late to
> impose limits only after returning to user mode?

Throttling is purely a CFS construct, and does not affect RT or DL
tasks (outside of the lock contention issues we're trying to fix :-)). If
an RT or DL task needs to run, it'll just preempt the CFS tasks, it won't
wait for any throttle or other mechanism.

> Furthermore, when a throttled
> task is awakened from S or D state, according to the logic of "imposing limits
> after returning to user mode," it could cause that low-priority task to wake
> up associated low-priority tasks one after another, leading to a sudden
> increase in running time, which contradicts the relatively precise CPU usage
> targets typically required in RT-Linux systems.

I would again say that's not a problem since those tasks are CFS.

> Additionally, I believe there are still many areas for improvement in my patch,
> and I hope to bring it to the community to gather suggestions from experts to
> see if there are areas for iterative improvement.
>
>
> Thanks
> Xin Zhao