Re: [PATCH 4/7] cgroup: rstat: support cgroup1

[Roman Gushchin <guro@fb.com>]

On Tue, Feb 02, 2021 at 01:47:43PM -0500, Johannes Weiner wrote:
> Rstat currently only supports the default hierarchy in cgroup2. In
> order to replace memcg's private stats infrastructure - used in both
> cgroup1 and cgroup2 - with rstat, the latter needs to support cgroup1.
> 
> The initialization and destruction callbacks for regular cgroups are
> already in place. Remove the cgroup_on_dfl() guards to handle cgroup1.
> 
> The initialization of the root cgroup is currently hardcoded to only
> handle cgrp_dfl_root.cgrp. Move those callbacks to cgroup_setup_root()
> and cgroup_destroy_root() to handle the default root as well as the
> various cgroup1 roots we may set up during mounting.
> 
> The linking of css to cgroups happens in code shared between cgroup1
> and cgroup2 as well. Simply remove the cgroup_on_dfl() guard.
> 
> Linkage of the root css to the root cgroup is a bit trickier: per
> default, the root css of a subsystem controller belongs to the default
> hierarchy (i.e. the cgroup2 root). When a controller is mounted in its
> cgroup1 version, the root css is stolen and moved to the cgroup1 root;
> on unmount, the css moves back to the default hierarchy. Annotate
> rebind_subsystems() to move the root css linkage along between roots.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Roman Gushchin <guro@fb.com>