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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 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. >