[systemd-devel] cgroupsv2 and realtime processes

[Michal Koutný]

Hello Michał.

On Sun, Jun 05, 2022 at 03:28:23PM +0200, Michał Zegan <webczat at outlook.com> wrote:
> I have kernel 5.17 on archlinux.

How is your kernel configured wrt CONFIG_RT_GROUP_SCHED?

> Is that still true?

That depends :-)

> Yet, checking /proc/(pid)/cgroup states these processes are not in a root
> cgroup, yet the cpu controller is enabled on the root cgroup
> (/sys/fs/cgroup/cgroup.subtree_control lists "cpu" as one of the controllers
> and I see the interface files in children).
> 
> Can anyone explain the situation?

With v2 and CONFIG_RT_GROUP_SCHED there's no way how to assign realtime
budgets to cgroups and therefore realtime tasks cannot run in them.

With !CONFIG_RT_GROUP_SCHED, there's (internally) only the root cgroup
for realtime tasks and things apparently work.

See also [1].

> <snip>
> The cgroupsv2 documentation states that cgroup cpu controller
> currently does not support realtime processes, so to enable it all
> realtime processes must be moved to root cgroup.

Will you send a docs patch with the CONFIG_RT_GROUP_SCHED reservation?
:-p

HTH,
Michal

[1] https://github.com/systemd/systemd/blob/369151c9c73b12fb7a88fc2b558499c2d4832982/README#L140