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Date:   Thu, 4 Feb 2021 11:01:30 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal Hocko <mhocko@suse.com>
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: <YBwaWkJgqPNF3I3w@cmpxchg.org>
References: <20210202184746.119084-1-hannes@cmpxchg.org>
 <20210202184746.119084-5-hannes@cmpxchg.org>
 <YBv5Dc1I9QpPH69n@dhcp22.suse.cz>
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On Thu, Feb 04, 2021 at 02:39:25PM +0100, Michal Hocko wrote:
> 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.

That's gone, right?

static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
                                      struct cftype *cft, u64 val)
{
        if (val == 1)
                return 0;

        pr_warn_once("Non-hierarchical mode is deprecated. "
                     "Please report your usecase to linux-mm@kvack.org if you "
                     "depend on this functionality.\n");

        return -EINVAL;
}