1CGROUP_NAMESPACES(7) Linux Programmer's Manual CGROUP_NAMESPACES(7)
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6 cgroup_namespaces - overview of Linux cgroup namespaces
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9 For an overview of namespaces, see namespaces(7).
10 Cgroup namespaces virtualize the view of a process's cgroups (see
12 cgroups(7)) as seen via /proc/[pid]/cgroup and /proc/[pid]/mountinfo.
13 Each cgroup namespace has its own set of cgroup root directories.
15 These root directories are the base points for the relative locations
16 displayed in the corresponding records in the /proc/[pid]/cgroup file.
17 When a process creates a new cgroup namespace using clone(2) or
18 unshare(2) with the CLONE_NEWCGROUP flag, it enters a new cgroup names‐
19 pace in which its current cgroups directories become the cgroup root
20 directories of the new namespace. (This applies both for the cgroups
21 version 1 hierarchies and the cgroups version 2 unified hierarchy.)
22 When viewing /proc/[pid]/cgroup, the pathname shown in the third field
24 of each record will be relative to the reading process's root directory
25 for the corresponding cgroup hierarchy. If the cgroup directory of the
26 target process lies outside the root directory of the reading process's
27 cgroup namespace, then the pathname will show ../ entries for each
28 ancestor level in the cgroup hierarchy.
29 The following shell session demonstrates the effect of creating a new
31 cgroup namespace. First, (as superuser) we create a child cgroup in
32 the freezer hierarchy, and put the shell into that cgroup:
33 # mkdir -p /sys/fs/cgroup/freezer/sub
35 # echo $$ # Show PID of this shell
36 30655
37 # sh -c 'echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs'
38 # cat /proc/self/cgroup | grep freezer
39 7:freezer:/sub
40 Next, we use unshare(1) to create a process running a new shell in new
42 cgroup and mount namespaces:
43 # unshare -Cm bash
45 We then inspect the /proc/[pid]/cgroup files of, respectively, the new
47 shell process started by the unshare(1) command, a process that is in
48 the original cgroup namespace (init, with PID 1), and a process in a
49 sibling cgroup (sub2):
50 $ cat /proc/self/cgroup | grep freezer
52 7:freezer:/
53 $ cat /proc/1/cgroup | grep freezer
54 7:freezer:/..
55 $ cat /proc/20124/cgroup | grep freezer
56 7:freezer:/../sub2
57 From the output of the first command, we see that the freezer cgroup
59 membership of the new shell (which is in the same cgroup as the initial
60 shell) is shown defined relative to the freezer cgroup root directory
61 that was established when the new cgroup namespace was created. (In
62 absolute terms, the new shell is in the /sub freezer cgroup, and the
63 root directory of the freezer cgroup hierarchy in the new cgroup names‐
64 pace is also /sub. Thus, the new shell's cgroup membership is dis‐
65 played as '/'.)
66 However, when we look in /proc/self/mountinfo we see the following
68 anomaly:
69 # cat /proc/self/mountinfo | grep freezer
71 155 145 0:32 /.. /sys/fs/cgroup/freezer ...
72 The fourth field of this line (/..) should show the directory in the