Add ability to set cgroup parent for all containers

Fix #18022

Signed-off-by: Alexander Morozov <lk4d4@docker.com>
Upstream-commit: 2e3186ab065a7130a6e6267d86750848d5eb7d80
Component: engine

components/engine/docs/reference/commandline/daemon.md

@@ -20,6 +20,7 @@ weight = -1
       --authz-plugin=[]                     Set authorization plugins to load
       -b, --bridge=""                        Attach containers to a network bridge
       --bip=""                               Specify network bridge IP
+      --cgroup-parent=/docker                Set parent cgroup for all containers
       -D, --debug                            Enable debug mode
       --default-gateway=""                   Container default gateway IPv4 address
       --default-gateway-v6=""                Container default gateway IPv6 address
@@ -643,4 +644,20 @@ set like this:
     /usr/local/bin/docker daemon -D -g /var/lib/docker -H unix:// > /var/lib/docker-machine/docker.log 2>&1
 
 
+# Default cgroup parent
 
+The `--cgroup-parent` option allows you to set the default cgroup parent
+to use for containers. If this option is not set, it defaults to `/docker`.
+
+If the cgroup has a leading forward slash (`/`), the cgroup is created
+under the root cgroup, otherwise the cgroup is created under the daemon
+cgroup.
+
+Assuming the daemon is running in cgroup `daemoncgroup`,
+`--cgroup-parent=/foobar` creates a cgroup in
+`/sys/fs/cgroup/memory/foobar`, wheras using `--cgroup-parent=foobar`
+creates the cgroup in `/sys/fs/cgroup/memory/daemoncgroup/foobar`
+
+This setting can also be set per container, using the `--cgroup-parent`
+option on `docker create` and `docker run`, and takes precedence over
+the `--cgroup-parent` option on the daemon.