[systemd-devel] Fedora 25, cgroups V2 and systemd roadmap

Daniel P. Berrange berrange at redhat.com
Mon Oct 10 11:47:18 UTC 2016

On Mon, Oct 10, 2016 at 02:31:55PM +0300, Kevin Wilson wrote:
> Hello, systemd developers,
> So we have now 3 V2 cgroups controller in the kernel (pids, memory and io).
> The CPU controller as of now is not merged in and is available only in
> an out of tree git repo (due to some debate over
> it with kernel scheduler developers). Not sure that it will be merged
> in the next 2 months.
> Fedora 25 is to be released in a month and a half, on 15 of November.
> https://fedoraproject.org/wiki/Releases/25/Schedule
> My questions are:
> what are the intentions regarding using cgroup v2 in systemd  in F25
> as the default instead of using cgroup V1?
> Is the absence of  the CPU controller is a reason for not having
> cgroup V2 as a default in F25 ? and if so, why ?

Ignoring the question of Fedora switching, more generally, if any OS were
to switch to cgroup v2 right now it would break a number of applications
that use cgroups v1 today. v2 is not a plain no-op drop-in replacement for
v1, as they have pretty different rules around the hierarchy management.
Applications that create/manage cgroups properties/dirs need to be manually
ported, not merely systemd itself. The absence of CPU controller support
would also be a functional regression for some applications, effectively
preventing use of cgroup v2 even if they were ported.

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