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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 23 Jun 2022 09:55:33 -0700
Message-ID: <CALvZod4WsmrpfZtfnP4AmvtnkvBRngXXGyhM6+aQzkdfjsinAQ@mail.gmail.com>
Subject: Re: [PATCH mm v5 0/9] memcg: accounting for objects allocated by
 mkdir, cgroup
To:     Michal Hocko <mhocko@suse.com>
Cc:     Vasily Averin <vvs@openvz.org>, kernel@openvz.org,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Muchun Song <songmuchun@bytedance.com>,
        Cgroups <cgroups@vger.kernel.org>
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On Thu, Jun 23, 2022 at 9:07 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 23-06-22 18:03:31, Vasily Averin wrote:
> > Dear Michal,
> > do you still have any concerns about this patch set?
>
> Yes, I do not think we have concluded this to be really necessary. IIRC
> Roman would like to see lingering cgroups addressed in not-so-distant
> future (http://lkml.kernel.org/r/Ypd2DW7id4M3KJJW@carbon) and we already
> have a limit for the number of cgroups in the tree. So why should we
> chase after allocations that correspond the cgroups and somehow try to
> cap their number via the memory consumption. This looks like something
> that will get out of sync eventually and it also doesn't seem like the
> best control to me (comparing to an explicit limit to prevent runaways).
> --

Let me give a counter argument to that. On a system running multiple
workloads, how can the admin come up with a sensible limit for the
number of cgroups? There will definitely be jobs that require much
more number of sub-cgroups. Asking the admins to dynamically tune
another tuneable is just asking for more complications. At the end all
the users would just set it to max.

I would recommend to see the commit ac7b79fd190b ("inotify, memcg:
account inotify instances to kmemcg") where there is already a sysctl
(inotify/max_user_instances) to limit the number of instances but
there was no sensible way to set that limit on a multi-tenant system.