Re: [RFC PATCH v5 20/29] sched/deadline: Allow deeper hierarchies of RT cgroups

[luca abeni]

Hi Tejun,

first of all, thanks for your comments! I think this is the kind of
dicussion that we need to have...
Right now we have something that works "well enough" for real-time, but
we want to make it useful in general, so that distributions will not
disable it by default.

I need to better study your suggestions (I do not know cgroup v2
much...), but I have some questions to better understand possible
solutions:

On Tue, 5 May 2026 09:56:58 -1000
Tejun Heo <tj@xxxxxxxxxx> wrote:
[...]
> - cgroup2 enforces that internal cgroups w/ controllers enabled
> cannot have threads in them. No need to enforce that separately.
> 
> - However, the cpu controller is a threaded controller which means
> that it can have threaded sub-hierarchy where the no-internal-process
> rule doesn't apply. This was created explicitly for cpu controller.
> The proposed change blocks it effectively forcing cpu controller into
> regular domain controller behavior subject to no-internal-process
> rule. Note these are enforced at controller granularity and this
> means that users who use the threaded mode will be forced to pick
> between the two.

Just to better understand: would it make sense to allow non-{FIFO,RT}
tasks to be in non-leaf cgroups (as allowed by the threaded CPU
controller), while enforcing that FIFO/RR tasks can only be in leaf
cgroups? Or would this be a hack that compromises the rt-CPU controller
usefulness?


> - This has the same problem with cgroup1's rt cgroup sched support
> where there is no way to have a permissive default configuration,
> which means that users who don't really care about distributing rt
> shares hierarchically would get blocked from running rt processes by
> default, which basically forces distros to disable rt cgroup sched
> support. This is not new but it'd be a shame to put in all the work
> and the end result is that most people don't even have access to the
> feature.

Yes, we have a bad default here.
Would a default like "allow running FIFO/RR tasks without runtime
enforcement" (this is what happens to FIFO/RR tasks running in the root
control group) be acceptable?


			Thanks,
				Luca

> 
> Here's my suggestion if there is desire for this to become something
> most people have easy access to:
> 
> - Don't make it impossible to use in conjunction with other resource
> control mechanisms especially not CPU controller itself. Don't force
> people to choose between threaded mode and rt control. Allow them to
> co-exist in a reasonable manner.
> 
> - The same in the wider scope. Don't let it get in the way of people
> who don't care about it. Compromising on interface / failure mode is
> better than people not being able to use it in most cases.
> 
> Thanks.
>