Make sure to enable memory and cpuset cgroup in v2

doc/manual/install-judgehost.rst

@@ -179,8 +179,8 @@ Optionally the timings can be made more stable by not letting the OS schedule
 any other tasks on the same CPU core the judgedaemon is using:
 ``GRUB_CMDLINE_LINUX_DEFAULT="quiet cgroup_enable=memory swapaccount=1 isolcpus=2"``
 
-You have now configured the system to use cgroups. On systems with
-cgroups v1, you need to run::
+You have now configured the system to use cgroups. To create
+the actual cgroups that DOMjudge will use you need to run::
 
   sudo systemctl enable create-cgroups --now
 

judge/create_cgroups.in

@@ -9,24 +9,27 @@
 JUDGEHOSTUSER=@DOMJUDGE_USER@
 CGROUPBASE="/sys/fs/cgroup"
 
-# We do not need to do any of this with cgroup v2.
-fs_type=$(awk '$2 == "/sys/fs/cgroup" {print $3}' /proc/mounts)
-if [ "$fs_type" = "cgroup2" ]; then
-    echo "cgroup v2 detected, skipping cgroup creation" >&2
-    exit 0
-fi
-
 cgroup_error_and_usage () {
     echo "$1" >&2
     echo "To fix this, please make the following changes:
     1. In /etc/default/grub, add 'cgroup_enable=memory swapaccount=1' to GRUB_CMDLINE_LINUX_DEFAULT.
-       On modern distros (e.g. Debian bullseye and Ubuntu Jammy Jellyfish) which have cgroup v2 enabled by default,
-       you need to add 'systemd.unified_cgroup_hierarchy=0' as well.
     2. Run update-grub
     3. Reboot" >&2
     exit 1
 }
 
+# Check whether cgroup v2 is available.
+fs_type=$(awk '$2 == "/sys/fs/cgroup" {print $3}' /proc/mounts)
+if [ "$fs_type" = "cgroup2" ]; then
+    if ! echo "+memory" >> /sys/fs/cgroup/cgroup.subtree_control; then
+	    cgroup_error_and_usage "Error: Cannot add +memory to cgroup.subtree_control; check kernel params. Unable to continue."
+    fi
+    if ! echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control; then
+	    cgroup_error_and_usage "Error: Cannot add +cpuset to cgroup.subtree_control; check kernel params. Unable to continue."
+    fi
+    exit 0
+fi
+
 for i in cpuset memory; do
     mkdir -p $CGROUPBASE/$i
     if [ ! -d $CGROUPBASE/$i/ ]; then