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Subject: Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some
 machines
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From: "Singh, Balbir" <bsingharora@gmail.com>
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Date: Thu, 26 Jul 2018 10:55:08 +1000
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To: Bruce Merry <bmerry@ska.ac.za>, Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>, Johannes Weiner <hannes@cmpxchg.org>, Vladimir Davydov <vdavydov.dev@gmail.com>



On 7/19/18 3:40 AM, Bruce Merry wrote:
> On 18 July 2018 at 17:49, Shakeel Butt <shakeelb@google.com> wrote:
>> On Wed, Jul 18, 2018 at 8:37 AM Bruce Merry <bmerry@ska.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?
> 

Do you by any chance have use_hierarch=1? memcg_stat_show should just rely on counters inside the memory cgroup and the the LRU sizes for each node.

Balbir Singh.