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

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Date:   Thu, 6 Aug 2020 21:16:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <guro@...com>, 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

On Wed, 29 Jul 2020 19:10:39 +0200 Michal Koutný <mkoutny@...e.com> wrote:

> 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.

These week-old issues appear to be significant.  Roman?  Or someone
else?