[systemd-devel] unable to attach pid to service delegated directory in unified mode after restart

[Michal Koutný]

On Tue, Mar 15, 2022 at 04:35:12PM +0100, Felip Moll <felip at schedmd.com> wrote:
> Meaning that it would be great to have a delegated cgroup subtree without
> the need of a service or scope.
> Just an empty subtree.

It looks appealing to add Delegate= directive to slice units.
Firstly, that'd prevent the use of the slice by anything systemd.
Then some notion of owner of that subtree would have to be defined (if
only for cleanup).
That owner would be a process -- bang, you created a service with
delegation or a scope with "keepalive" process.

(The above is slightly misleading) there could be an alternative of
something like RemainAfterExit=yes for scopes, i.e. such scopes would
not be stopped after last process exiting (but systemd would still be in
charge of cleaning the cgroup after explicit stop request and that'd
also mark the scope as truly stopped).
Such a recycled scope would only be useful via
org.freedesktop.systemd1.Manager.AttachProcessesToUnit().

BTW I'm also wondering how do you detect a job finishing in the case
original parent is gone (due to main service restart) and job's main
process reparented?

BTW 2 You didn't like having a scope for each job. Is it because of the
setup time (IOW jobs are short-lived) or persistent scopes overhead (too
many units, PID1 scalability)?

Michal