The percpu memory used by memcg cannot be cleared

[Liu Jing <liujing@cmss.chinamobile.com>]

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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。

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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


step3：view /proc/cgroup and /proc/meminfo  files

[root@localhost ~]# cat /proc/cgroups
#subsys_name    hierarchy       num_cgroups     enabled
cpuset           10                   1           1
cpu              4                     1           1
cpuacct         4                     1           1
blkio            13                     1          1
memory          14                 2009             1
devices         6                   94             1

[root@localhost ~]# cat /proc/meminfo | grep Percpu
Percpu:           600576 kB


step4：when I use "echo 0 > /sys/fs/cgroup/memory/user.slice/user-0.slice/memory.force_empty", I find the num_cgroups of memory and percpu  have no changed

[root@localhost ~]# echo 0 > /sys/fs/cgroup/memory/user.slice/user-0.slice/memory.force_empty
[root@localhost ~]# cat /proc/cgroups
#subsys_name    hierarchy       num_cgroups     enabled
cpuset              10               1            1
cpu                 4                 1           1
cpuacct              4               1             1
blkio                 13              1             1
memory              14              2039            1
devices             6               87               1

[root@localhost ~]# cat /proc/meminfo | grep Percpu
Percpu:           600576 kB


step 5: when I use swapon -a to open swap, then echo 0 > /sys/fs/cgroup/memory/user.slice/user-0.slice/memory.force_empty again

[root@localhost ~]# swapon -a
[root@localhost ~]# echo 0 > /sys/fs/cgroup/memory/user.slice/user-0.slice/memory.force_empty


step 6: view /proc/cgroup and /proc/meminfo  files ,I found the the num_cgroups of memory and percpu  have been reduced. 
[root@localhost ~]# cat /proc/cgroups
#subsys_name    hierarchy       num_cgroups     enabled
cpuset              10             1                 1
cpu               4                1                 1
cpuacct            4                1                  1
blkio            13                1                  1
memory            14              185                 1
devices           6               87                 1
freezer           9                1                 1

[root@localhost ~]# cat /proc/meminfo | grep Percpu
Percpu:           120832 kB
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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. 
I want to know why ,I look forward to hearing back from the community.
Thanks.