linux-kernel - Re: [RFC PATCH v3 14/24] sched/rt: Allow zeroing the runtime of the root control group

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Message-ID: <aOe-fqaL3uP6ukCq@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 9 Oct 2025 15:54:06 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Yuri Andriaccio <yurand2000@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Yuri Andriaccio <yuri.andriaccio@...tannapisa.it>
Subject: Re: [RFC PATCH v3 14/24] sched/rt: Allow zeroing the runtime of the
 root control group

Hello,

On 29/09/25 11:22, Yuri Andriaccio wrote:
> In HCBS, the root control group follows the already existing rules for rt-task
> scheduling. As such, it does not make use of the deadline servers to account for
> runtime, or any other HCBS specific code and features.
> 
> While the runtime of SCHED_DEADLINE tasks depends on the global bandwidth
> reserved for rt_tasks, the runtime of SCHED_FIFO/SCHED_RR tasks is limited by
> the activation of fair-servers (as the RT_THROTTLING mechanism has been removed
> in favour of them), thus their maximum bandwidth depends solely on the
> fair-server settings (which are thightly related to the global bandwdith
                                                                    ^
							      bandwidth

> reserved for rt-tasks) and the amount of SCHED_OTHER workload to run (recall
> that if no SCHED_OTHER tasks are running, the FIFO/RR tasks may fully utilize
> the CPU).
> 
> The values of runtime and period in the root cgroup's cpu controller do not
> affect, by design of HCBS, the fair-server settings and similar (consequently
> they do not affect the scheduling of FIFO/RR tasks in the root cgroup), buy they
                                                                           ^
									 but

> are just used to reserve a portion of the SCHED_DEADLINE bandwidth to the
> scheduling of rt-cgroups. These values only affect child cgroups, their deadline
> servers and their assigned FIFO/RR tasks.
> 
> Signed-off-by: Yuri Andriaccio <yurand2000@...il.com>
> ---

Thanks,
Juri