linux-kernel - Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup

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Date:   Wed, 29 Jul 2020 19:10:39 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the
 parent cgroup

Hello.

On Tue, Jun 23, 2020 at 11:45:14AM -0700, Roman Gushchin <guro@...com> wrote:
> Because the size of memory cgroup internal structures can dramatically
> exceed the size of object or page which is pinning it in the memory, it's
> not a good idea to simple ignore it.  It actually breaks the isolation
> between cgroups.
No doubt about accounting the memory if it's significant amount.

> Let's account the consumed percpu memory to the parent cgroup.
Why did you choose charging to the parent of the created cgroup?

Should the charge go the cgroup _that is creating_ the new memcg?

One reason is that there are the throttling mechanisms for memory limits
and those are better exercised when the actor and its memory artefact
are the same cgroup, aren't they?

The second reason is based on the example Dlegation Containment
(Documentation/admin-guide/cgroup-v2.rst)

> For an example, let's assume cgroups C0 and C1 have been delegated to
> user U0 who created C00, C01 under C0 and C10 under C1 as follows and
> all processes under C0 and C1 belong to U0::
> 
>   ~~~~~~~~~~~~~ - C0 - C00
>   ~ cgroup    ~      \ C01
>   ~ hierarchy ~
>   ~~~~~~~~~~~~~ - C1 - C10

Thanks to permissions a task running in C0 creating a cgroup in C1 would
deplete C1's supply victimizing tasks inside C1.

Thanks,
Michal

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