linux-kernel - Re: [RFC PATCH] memcg: use root_mem

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Message-ID: <YwXYVjRpXQjQMsxr@dhcp22.suse.cz>
Date:   Wed, 24 Aug 2022 09:50:46 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Ke Wang <ke.wang@...soc.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [RFC PATCH] memcg: use root_mem_cgroup when css is inherited

On Wed 24-08-22 10:23:14, Zhaoyang Huang wrote:
> On Tue, Aug 23, 2022 at 7:51 PM Michal Hocko <mhocko@...e.com> wrote:
[...]
> > One way to achieve that would be shaping the hierarchy the following way
> >             root
> >         /         \
> > no_memcg[1]      memcg[2]
> > ||||||||         |||||
> > app_cgroups     app_cgroups
> >
> > with
> > no_memcg.subtree_control = ""
> > memcg.subtree_control = memory
> >
> > no?
> According to my understanding, No as there will be no no_memcg. All
> children groups under root would have its cgroup.controllers = memory
> as long as root has memory enabled.

Correct

> Under this circumstance, all
> descendants group under 'no_memcg' will charge memory to its parent
> group.

Correct. And why is that a problem? I thought you main concern was a per
application LRUs. With the above configuration all app_cgroups which do
not require an explicit memory control will share the same (no_memcg)
LRU and they will be aged together.
-- 
Michal Hocko
SUSE Labs