linux-kernel - Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj

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Message-ID: <87pn5irz2m.fsf@suse.de>
Date:   Fri, 16 Oct 2020 16:05:21 +0100
From:   Richard Palethorpe <rpalethorpe@...e.de>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <guro@...com>, ltp@...ts.linux.it,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root

Hello,

Richard Palethorpe <rpalethorpe@...e.de> writes:

> Hello Michal,
>
> Michal Koutný <mkoutny@...e.com> writes:
>
>> Hello.
>>
>> On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <rpalethorpe@...e.com> wrote:
>>> SLAB objects which outlive their memcg are moved to their parent
>>> memcg where they may be uncharged. However if they are moved to the
>>> root memcg, uncharging will result in negative page counter values as
>>> root has no page counters.
>> Why do you think those are reparented objects? If those are originally
>> charged in a non-root cgroup, then the charge value should be propagated up the
>> hierarchy, including root memcg, so if they're later uncharged in root
>> after reparenting, it should still break even. (Or did I miss some stock
>> imbalance?)
>
> I traced it and can see they are reparented objects and that the root
> groups counters are zero (or negative if I run madvise06 multiple times)
> before a drain takes place. I'm guessing this is because the root group
> has 'use_hierachy' set to false so that the childs page_counter parents
> are set to NULL. However I will check, because I'm not sure about
> either.

Yes, it appears that use_hierarchy=0 which is probably because the test
mounts cgroup v1, creates a child group within that and does not set
use_hierarchy on the root. On v2 root always has use_hierarchy enabled.

>
>>
>> (But the patch seems justifiable to me as objects (not)charged directly to
>> root memcg may be incorrectly uncharged.)
>>
>> Thanks,
>> Michal

I'm don't know if that could happen without reparenting. I suppose if
use_hierarchy=1 then actually this patch will result in root being
overcharged, so perhaps it should also check for use_hierarchy?

-- 
Thank you,
Richard.