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Date:   Thu, 4 Feb 2021 14:39:25 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Roman Gushchin <guro@fb.com>,
        linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 4/7] cgroup: rstat: support cgroup1
Message-ID: <YBv5Dc1I9QpPH69n@dhcp22.suse.cz>
References: <20210202184746.119084-1-hannes@cmpxchg.org>
 <20210202184746.119084-5-hannes@cmpxchg.org>
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On Tue 02-02-21 13:47:43, 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.

I am not familiar with rstat API and from this patch it is not really
clear to me how does it deal with memcg v1 use_hierarchy oddness.
-- 
Michal Hocko
SUSE Labs