linux-kernel - Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

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Date:   Wed, 18 Jul 2018 11:13:12 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     bmerry@....ac.za
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

On Wed, Jul 18, 2018 at 10:58 AM Bruce Merry <bmerry@....ac.za> wrote:
>
> On 18 July 2018 at 19:48, Shakeel Butt <shakeelb@...gle.com> wrote:
> > On Wed, Jul 18, 2018 at 10:40 AM Bruce Merry <bmerry@....ac.za> wrote:
> >> > Yes, very easy to produce zombies, though I don't think kernel
> >> > provides any way to tell how many zombies exist on the system.
> >> >
> >> > To create a zombie, first create a memcg node, enter that memcg,
> >> > create a tmpfs file of few KiBs, exit the memcg and rmdir the memcg.
> >> > That memcg will be a zombie until you delete that tmpfs file.
> >>
> >> Thanks, that makes sense. I'll see if I can reproduce the issue. Do
> >> you expect the same thing to happen with normal (non-tmpfs) files that
> >> are sitting in the page cache, and/or dentries?
> >>
> >
> > Normal files and their dentries can get reclaimed while tmpfs will
> > stick and even if the data of tmpfs goes to swap, the kmem related to
> > tmpfs files will remain in memory.
>
> Sure, page cache and dentries are reclaimable given memory pressure.
> These machines all have more memory than they need though (64GB+) and
> generally don't come under any memory pressure. I'm just wondering if
> the behaviour we're seeing can be explained as a result of a lot of
> dentries sticking around (because there is no memory pressure) and in
> turn causing a lot of zombie cgroups to stay present until something
> forces reclamation of dentries.
>

Yes, if there is no memory pressure such memory can stay around.

On your production machine, before deleting memory containers, you can
try force_empty to reclaim such memory from them. See if that helps.

Shakeel