Comment 1 for bug 1844879

The commit where the regression is introduced in this commit:

commit 0219b3524f414e23589e63c6de6a759811ef8474
Author: Donald Buczek <email address hidden>
Date: Thu Apr 25 09:39:41 2019 +0200

    cgroup: Continue unit reset if cgroup is busy

    When part of the cgroup hierarchy cannot be deleted (e.g. because there
    are still processes in it), do not exit unit_prune_cgroup early, but
    continue so that u->cgroup_realized is reset.

    Log the known case of non-empty cgroups at debug level and other errors
    at warning level.

    Fixes https://github.com/systemd/systemd/issues/12386