moby: Docker in Docker - failed to find the cgroup root

cgroupfs_mount() {
        # see also https://github.com/tianon/cgroupfs-mount/blob/master/cgroupfs-mount
        if grep -v '^#' /etc/fstab | grep -q cgroup \
                || [ ! -e /proc/cgroups ] \
                || [ ! -d /sys/fs/cgroup ]; then
                return
        fi
        if ! mountpoint -q /sys/fs/cgroup; then
                mount -t tmpfs -o uid=0,gid=0,mode=0755 cgroup /sys/fs/cgroup
        fi
        (
                cd /sys/fs/cgroup
                for sys in $(awk '!/^#/ { if ($4 == 1) print $1 }' /proc/cgroups); do
                        mkdir -p $sys
                        if ! mountpoint -q $sys; then
                                if ! mount -n -t cgroup -o $sys cgroup $sys; then
                                        rmdir $sys || true
                                fi
                        fi
                done
        )
}

~                                                                                                                                                                                                           
~                                                                                                                                                                                                           
root@7737ff9e148f:~# source cgroup.sh 
root@7737ff9e148f:~# cgroup 
cgroupfs_mount  cgroups-mount   cgroups-umount  
root@7737ff9e148f:~# cgroupfs_mount 
mount: cgroup already mounted or cpu busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or cpuacct busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or net_cls busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or net_prio busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
root@7737ff9e148f:~# mount
/dev/sdb on / type btrfs (rw,relatime,space_cache)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev type tmpfs (rw,nosuid,mode=755)
shm on /dev/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=65536k)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
/dev/sdb on /etc/resolv.conf type btrfs (rw,relatime,space_cache)
/dev/sdb on /etc/hostname type btrfs (rw,relatime,space_cache)
/dev/sdb on /etc/hosts type btrfs (rw,relatime,space_cache)
/dev/sdb on /var/lib/docker type btrfs (rw,relatime,space_cache)
cgroup on /sys/fs/cgroup type tmpfs (rw,relatime,mode=755)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,relatime,memory)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,relatime,devices)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,relatime,freezer)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,relatime,blkio)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,relatime,perf_event)
/dev/sdb on /var/lib/docker/btrfs type btrfs (rw,relatime,space_cache)
root@7737ff9e148f:~# umount /sys/fs/cgroup/cpu
umount: /sys/fs/cgroup/cpu: not found
root@7737ff9e148f:~# cgroupfs_mount           
mount: cgroup already mounted or cpu busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or cpuacct busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or net_cls busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup
mount: cgroup already mounted or net_prio busy
mount: according to mtab, cgroup is mounted on /sys/fs/cgroup

This may help in debugging. Host docker is CoreOS 472 and guest docker is dockerhub:flitter/builder:master

For anyone who may be having or is still having this issue on a RHEL server, would you be able to verify if the libcgroup package is installed using sudo yum list installed libcgroup? If it isn’t try installing it (sudo yum install libcgroup).

Assuming it is installed already, first of all, check the status:

$ sudo /etc/init.d/cgconfig status
Running

If it is stopped, start it:

$ sudo /etc/init.d/cgconfig start
Starting cgconfig service:                                 [  OK  ]

If it is already running, restart it:

$ sudo /etc/init.d/cgconfig restart
Stopping cgconfig service:                                 [  OK  ]
Starting cgconfig service:                                 [  OK  ]

The docker init script /etc/init.d/docker requires that the cgconfig service be started but it sounds like one (or more) of the cgroup filesystems becomes unconfigured for whatever reason after the service is started so restarting it ought to resolve the issues.

If it doesn’t, could someone post their /etc/cgconfig.conf and check to see if there are any files in /etc/cgconfig.d?

Thanks for this - this made docker-in-docker work for me:

https://github.com/ianmiell/shutit/commit/3d7238f02c23c40a34c5000c4e88f64565c3685c#diff-469a2a5ebd309b7bea7966e84b092112

Sending you a virtual beer/coffee/the purest water.