Re: Realtime throttling and Real-Time group scheduling

[John Ogness <john.ogness@linutronix.de>]

On 2025-10-21, KHUENY.Gerhard <Gerhard.KHUENY@bachmann.info> wrote:
>> > I'm running a 6.12.39-rt Linux kernel and have been experimenting with
>> > runtime throttling on my embedded hardware.  However, it seems that
>> > the settings for sched_rt_period_us and sched_rt_runtime_us have no
>> > effect. In my kernel configuration, CONFIG_RT_GROUP_SCHED is not set.
>> >
>> > Is my understanding correct that CONFIG_RT_GROUP_SCHED is not required
>> > to throttle real-time tasks?
>> 
>> Yes. But instead of adjusting "throttling", you adjust the guaranteed
>> bandwidth for !rt tasks. You can read about the fair deadline server in its
>> original commit message [0].
>> 
>> Basically you can adjust these debugfs files:
>> 
>> /sys/kernel/debug/sched/fair_server/cpu*/runtime
>> 
>> to guarantee bandwith for !rt tasks.
>> 
>> And yes, we are aware that proper documentation for this is missing.
>> 
>> John Ogness
>> 
>> [0]
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/Do
>> cumentation?&id=a110a81c52a9de73e2e57e826dd3bf0fd4c22226
>
> Thanks for clarification. So this means that if CONFIG_RT_GROUP_SCHED
> is not set, the parameters /proc/sys/kernel/sched_rt_period and
> /proc/sys/kernel/sched_rt_runtime have no effect?

They have no effect on the "RT throttling" feature, which does not exist
for CONFIG_RT_GROUP_SCHED=n. See where it was removed here [1].

However, the parameters are still used for admission control of
SCHED_DEADLINE tasks. And likewise they also affect what values can be
written to /sys/kernel/debug/sched/fair_server/cpu*/runtime to configure
the fair deadline server.

Note that this is all new starting with v6.12.

John Ogness

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5f6bd380c7bdbe10f7b4e8ddcceed60ce0714c6d