linux-kernel - Re: [PATCH] mm: introduce entrance for root_mem

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Message-ID: <Y9zM0GbmfKdvy3bL@dhcp22.suse.cz>
Date:   Fri, 3 Feb 2023 09:58:56 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Zhaoyang Huang <huangzhaoyang@...il.com>, ke.wang@...soc.com
Subject: Re: [PATCH] mm: introduce entrance for root_mem_cgroup's current

On Thu 02-02-23 10:24:08, Shakeel Butt wrote:
> On Thu, Feb 2, 2023 at 12:27 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 02-02-23 12:32:57, zhaoyang.huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > >
> > > Introducing memory.root_current for the memory charges on root_mem_cgroup.
> >
> > Charges are not currently accounted for the root memcg universally. See
> > try_charge which is used for all user space and skmem charges. I am not
> > 100% sure about objcg based accounting because there is no explicit
> > check for the root memcg but this might be hidden somewhere as well.
> 
> Yes in __get_obj_cgroup_from_memcg(). However the reason to use
> try_charge_memcg() to bypass root check was to avoid the race with
> reparenting. More details in c5c8b16b596e ("mm: memcontrol: fix
> root_mem_cgroup charging").

Thanks for the pointer!

> > That means that the patch as is doesn't really provide and usable value.
> > The root exemption has been removed in the past but that has been
> > reverted due to a regression. See ce00a967377b ("mm: memcontrol: revert
> > use of root_mem_cgroup res_counter") for more.
> >
> 
> One advantage I can see is if someone is looking for usage for all top
> containers (alive or zombie) but I wanted to know if that was the real
> motivation behind the patch.

Isn't that just a global stats that we already display via /proc files?

-- 
Michal Hocko
SUSE Labs