[systemd-devel] Feedback sought: can we drop cgroupv1 support soon?

[Christian Brauner]

[Cc Stéphane and Serge]

On Thu, Jul 21, 2022 at 11:03:49AM +0200, Lennart Poettering wrote:
> Heya!
> 
> It's currently a terrible mess having to support both cgroupsv1 and
> cgroupsv2 in our codebase.
> 
> cgroupsv2 first entered the kernel in 2014, i.e. *eight* years ago
> (kernel 3.16). We soon intend to raise the baseline for systemd to
> kernel 4.3 (because we want to be able to rely on the existance of
> ambient capabilities), but that also means, that all kernels we intend
> to support have a well-enough working cgroupv2 implementation.
> 
> hence, i'd love to drop the cgroupv1 support from our tree entirely,
> and simplify and modernize our codebase to go cgroupv2-only. Before we
> do that I'd like to seek feedback on this though, given this is not
> purely a thing between the kernel and systemd — this does leak into
> some userspace, that operates on cgroups directly.
> 
> Specifically, legacy container infra (i.e. docker/moby) for the
> longest time was cgroupsv1-only. But as I understand it has since been
> updated, to cgroupsv2 too.
> 
> Hence my question: is there a strong community of people who insist on
> using newest systemd while using legacy container infra? Anyone else
> has a good reason to stick with cgroupsv1 but really wants newest
> systemd?
> 
> The time where we'll drop cgroupv1 support *will* come eventually
> either way, but what's still up for discussion is to determine
> precisely when. hence, please let us know!

In general, I wouldn't mind dropping cgroup1 support in the future.

The only thing I immediately kept thinking about is what happens to
workloads that have a v1 cgroup layout on the host possibly with an
older systemd running container workloads using a newer distro with a
systemd version without cgroup1 support.

Think Ubuntu 18.04 host running a really new Ubuntu LTS that has a
version of systemd with cgroup1 support already dropped. People do
actually do stuff like that. Stéphane and Serge might know more about
actual use-cases in that area.

But fwiw, we did have people show up with this and related problems for
the last 5 years or so at conferences.

Christian