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

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Message-ID: <20180725123237.GI28386@dhcp22.suse.cz>
Date:   Wed, 25 Jul 2018 14:32:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Bruce Merry <bmerry@....ac.za>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        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 Tue 24-07-18 12:05:35, Bruce Merry wrote:
[...]
> I've also added some code to memcg_stat_show to report the number of
> cgroups in the hierarchy (iterations in for_each_mem_cgroup_tree).
> Running the script increases it from ~700 to ~41000. The script
> iterates 250,000 times, so only some fraction of the cgroups become
> zombies.

So this is definitely "too many zombies" to delay the collecting of
cumulative stats. Maybe we need to limit the number of zombies and
reclaim them more actively. I have seen Shakeel has posted something but
it looked more on the accounting side from a quick glance.

I can see you are using cgroup v1 so your workaround would be to
memory.force_empty before you remove the group.

-- 
Michal Hocko
SUSE Labs