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

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Date:   Wed, 18 Jul 2018 08:49:09 -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 8:37 AM Bruce Merry <bmerry@....ac.za> wrote:
>
> On 18 July 2018 at 17:26, Shakeel Butt <shakeelb@...gle.com> wrote:
> > On Wed, Jul 18, 2018 at 7:29 AM Bruce Merry <bmerry@....ac.za> wrote:
> > It seems like you are using cgroup-v1. How many nodes are there in
> > your memcg tree and also how many cpus does the system have?
>
> From my original email: "there are 106 memory.stat files in
> /sys/fs/cgroup/memory." - is that what you mean by the number of
> nodes?

Yes but it seems like your system might be suffering with zombies.

>
> The affected systems all have 8 CPU cores (hyperthreading is disabled).
>
> > Please note that memcg_stat_show or reading memory.stat in cgroup-v1
> > is not optimized as cgroup-v2. The function memcg_stat_show() in 4.13
> > does ~17 tree walks and then for ~12 of those tree walks, it goes
> > through all cpus for each node in the memcg tree. In 4.16,
> > a983b5ebee57 ("mm: memcontrol: fix excessive complexity in memory.stat
> > reporting") optimizes aways the cpu traversal at the expense of some
> > accuracy. Next optimization would be to do just one memcg tree
> > traversal similar to cgroup-v2's memory_stat_show().
>
> On most machines it is still fast (1-2ms), and there is no difference
> in the number of CPUs and only very small differences in the number of
> live memory cgroups, so presumably something else is going on.
>
> > The memcg tree does include all zombie memcgs and these zombies does
> > contribute to the memcg_stat_show cost.
>
> That sounds promising. Is there any way to tell how many zombies there
> are, and is there any way to deliberately create zombies? If I can
> produce zombies that might give me a reliable way to reproduce the
> problem, which could then sensibly be tested against newer kernel
> versions.
>

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.

Shakeel