linux-kernel - Re: [RFC/PATCH] sched: Support moving kthreads into cpuset cgroups

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Message-ID: <avxk2p2dr3bywzhujwnvbakjyv4gsnshssvgwj5276aojh7qbl@llhdz2e55iai>
Date: Mon, 12 May 2025 12:36:44 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Xi Wang <xii@...gle.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, David Rientjes <rientjes@...gle.com>, 
	Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, 
	Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Lai Jiangshan <jiangshanlai@...il.com1>, Frederic Weisbecker <frederic@...nel.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Dan Carpenter <dan.carpenter@...aro.org>, 
	Chen Yu <yu.c.chen@...el.com>, Kees Cook <kees@...nel.org>, Yu-Chun Lin <eleanor15x@...il.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Mickaël Salaün <mic@...ikod.net>
Subject: Re: [RFC/PATCH] sched: Support moving kthreads into cpuset cgroups

Hello.

On Tue, May 06, 2025 at 11:35:32AM -0700, Xi Wang <xii@...gle.com> wrote:
> In theory we should be able to manage kernel tasks with cpuset
> cgroups just like user tasks, would be a flexible way to limit
> interferences to real-time and other sensitive workloads.

I can see that this might be good for PF_USER_WORKER type of kernel
tasks. However, generic kernel tasks are spawned by kernel who
knows/demands which should run where and therefore they should not be
subject to cpuset restrictions. When limiting interference is
considered, there's CPU isolation for that.

The migratable kthreadd seems too coarse grained approach to me (also
when compared with CPU isolation).
I'd mostly echo Tejun's comment [1].

Regards,
Michal

[1] https://lore.kernel.org/r/aBqmmtST-_9oM9rF@slm.duckdns.org/

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