linux-kernel - Re: [RFC PATCH] memcg: adjust memcg for new cgroup allocations

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Date:   Tue, 23 Aug 2022 15:04:31 +0300
From:   Vasily Averin <vvs@...nvz.org>
To:     Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        gregkh@...uxfoundation.org, hannes@...xchg.org, kernel@...nvz.org,
        linux-kernel@...r.kernel.org, mhocko@...e.com, shakeelb@...gle.com,
        songmuchun@...edance.com, viro@...iv.linux.org.uk,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [RFC PATCH] memcg: adjust memcg for new cgroup allocations

On 8/17/22 19:41, Tejun Heo wrote:
> Hello,
> 
> On Wed, Aug 17, 2022 at 11:17:28AM +0200, Michal Koutný wrote:
>> On Wed, Aug 17, 2022 at 10:42:40AM +0300, Vasily Averin <vvs@...nvz.org> wrote:
>>> However, now we want to enable accounting for some other cgroup-related
>>> resources called from cgroup_mkdir. We would like to guarantee that
>>> all new accounted allocation will be charged to the same memory cgroup.
>>
>> Here's my point -- the change in the referenced patch applied to memory
>> controller hierarchies. This extension applies to any hierarchy that can
>> create groups, namely, a hierarchy without memory controller too. There
>> mem_cgroup_from_cgroup falls back to the root memcg (on a different
>> hierarchy).

My goal was to properly account kernfs and simple_xattr entries only, 
however I missed that it does not work in cgroup1 case.

>> If the purpose is to prevent unlimited creation of cgroup objects, the
>> root memcg is by principle unlimited, so it's just for accounting.

No, the goal is not to prevent unlimited creation of cgroup objects.
As Michal Hocko pointed it can be done via cgroup.max.descendants limits.

>> But I understand the purpose is to have everything under one roof,
>> unless the object lifetime is not bound to that owning memcg. Should
>> memory-less hierarchies be treated specially?
> 
> At least from my POV, as long as cgroup1 is not being regressed, we want to
> make decisions which make the best long term sense. We surely can
> accommodate cgroup1 as long as the added complexity is minimal but the bar
> is pretty high there. cgroup1 has been in maintenance mode for years now and
> even the basic delegation model isn't well established in cgroup1, so if we
> end up accounting everything in the root cgroup for most of cgroup1
> hierarchies, that sounds fine to me.

I would like to properly handle cgroup1 case too.
To do it we can enable accounting for new 'struct cgroup' objects, 
and bind them to memcg of creator task.

Thank you,
	Vasily Averin