linux-kernel - Re: [PATCH] mm, memcg: Report number of memcg caches in slabinfo

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Date:   Tue, 18 Jun 2019 14:37:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH] mm, memcg: Report number of memcg caches in slabinfo

On Mon 17-06-19 10:50:23, Waiman Long wrote:
> On 6/17/19 10:38 AM, Michal Hocko wrote:
> > [Cc linux-api]
> >
> > On Mon 17-06-19 10:21:49, Waiman Long wrote:
> >> There are concerns about memory leaks from extensive use of memory
> >> cgroups as each memory cgroup creates its own set of kmem caches. There
> >> is a possiblity that the memcg kmem caches may remain even after the
> >> memory cgroup removal.
> >>
> >> Therefore, it will be useful to show how many memcg caches are present
> >> for each of the kmem caches.
> > How is a user going to use that information?  Btw. Don't we have an
> > interface to display the number of (dead) cgroups?
> 
> The interface to report dead cgroups is for cgroup v2 (cgroup.stat)
> only. I don't think there is a way to find that for cgroup v1.

Doesn't debug_legacy_files provide the information for both cgroups
APIs?

> Also the
> number of memcg kmem caches may not be the same as the number of
> memcg's. It can range from 0 to above the number of memcg's.  So it is
> an interesting number by itself.

Is this useful enough to put into slabinfo? Doesn't this sound more like
a debugfs kinda a thing?

> From the user perspective, if the numbers is way above the number of
> memcg's, there is probably something wrong there.

-- 
Michal Hocko
SUSE Labs