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

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Date:   Wed, 18 Jul 2018 19:40:20 +0200
From:   Bruce Merry <bmerry@....ac.za>
To:     Shakeel Butt <shakeelb@...gle.com>
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 18 July 2018 at 17:49, Shakeel Butt <shakeelb@...gle.com> wrote:
> On Wed, Jul 18, 2018 at 8:37 AM Bruce Merry <bmerry@....ac.za> wrote:
>> 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.

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?

Cheers
Bruce
-- 
Bruce Merry
Senior Science Processing Developer
SKA South Africa