linux-kernel - Re: [PATCH] sched/core: Report failed rt migrations to non-root cgroup without rt bandwidth under RT_GROUP

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Message-ID: <ziakoghx6xbfvcocc5kcrcw5gv4xlphto44bptadkfbbtyf6op@d2fvj7rbec26>
Date: Tue, 16 Sep 2025 12:57:04 -0400
From: Aaron Tomlin <atomlin@...mlin.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: 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, vschneid@...hat.com, 
	shashank.mahadasyam@...y.com, longman@...hat.com, tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: Report failed rt migrations to non-root
 cgroup without rt bandwidth under RT_GROUP_SCHED

On Tue, Sep 16, 2025 at 05:44:14PM +0200, Michal Koutný wrote:
> On Tue, Sep 16, 2025 at 10:47:56AM -0400, Aaron Tomlin <atomlin@...mlin.com> wrote:
> > I would prefer not to take any action. However, is there a strong
> > preference to demote the rt task so the CPU controller can be enabled in
> > this context?
> 
> Maybe more context clarifies. The preference is not to end up in this
> corner.

I see. If I understand correctly, in this context, are you suggesting to
modify the identified task's scheduling class (e.g. to SCHED_NORMAL) so the
CPU controller can be enabled?

> I wonder does this combination come from a distro or is it your custom
> setup? I assume the latter (but I'm curious if there's such a distro), in
> that case you likely want to have the cpu controller on v1 hierarchy. v1
> usage is what the boottime switch is currently useful for, v2 de facto
> doesn't support RT group scheduling as of today [1], v2 systems should
> simply unset CONFIG_RT_GROUP_SCHED to avoid issues enabling cpu
> controller.

Yes. Under Red Hat Enterprise Linux (RHEL) 9 Kconfig option
CONFIG_RT_GROUP_SCHED and Cgroup version 2 is enabled by default.
Albeit, upstream can disable SCHED_FIFO/SCHED_RT group scheduling at
boot-time via rt_group_sched=.

> HTH,
> Michal
> 
> [1] This [2] didn't make it into tree thus I'd be reserved with the
>     message printed in your patch too.
> [2] https://lore.kernel.org/all/20250310170442.504716-11-mkoutny@suse.com/

I see. Understood.


Kind regards,
-- 
Aaron Tomlin