linux-kernel - Re: The percpu memory used by memcg cannot be cleared

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Message-ID: <ZtFv5jDx3Y3NMPrH@tiehlicka>
Date: Fri, 30 Aug 2024 09:08:22 +0200
From: Michal Hocko <mhocko@...e.com>
To: Liu Jing <liujing@...s.chinamobile.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: The percpu memory used by memcg cannot be cleared

On Fri 30-08-24 01:06:44, Liu Jing wrote:
> hello，linux boss
> 
>         I found a problem in the process of using linux memcg，When I turned swap off, the memcg memory I created with the following script could not be deleted with echo 0 > memory.force_empty, as explained below。
> 
> ----------------------------------------------------------------------------------------------------------
> step1：swapoff -a
> 
> 
> step2：use this script to create memcg
> 
> #!/bin/bash
> mkdir -p /tmp/test
> for i in 'seq 2000'
> do
>         sudo mkdir -p /sys/fs/cgroup/memory/user.slice/user-0.slice/test$ {i}
>         sudo echo $$ > /sys/fs/cgroup/memory/user.slice/user-0.slice/test$ {i}/tasks
>         sudo echo 'data' > /tmp/test/test$ {i}
>         sudo echo $$ > /sys/fs/cgroup/memory/user.slice/user-0.slice/tasks
>         sudo rmdir /sys/fs/cgroup/memory/user.slice/user-0.slice/test$ {i}
> done
[...]

I assume you /tmp is tmpfs backed.

> Therefore, I want to know why swap affects memcg memory reclamation,
> echo 0 > memory.force_empty this interface should force the memory
> used by the cgroup to be reclaimed. 

If the above is true then you simply do not have any backing storage to
reclaim to. You need swap to reclaim tmpfs/shmem memory.

-- 
Michal Hocko
SUSE Labs