Fall back to cgroup V1 if kernel does not support memory.peak in V2

and document kernel version requirement for cgroup V2.

Author: eldering

doc/manual/install-judgehost.rst

@@ -179,6 +179,12 @@ 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"``
 
+On modern systems where cgroup v2 is available, DOMjudge will try to
+use that. This requires kernel versions 5.20 or 6.0 or later to
+support reporting peak memory usage. If not found, the system will try
+to fall back to cgroup v1, but this might require you to add
+``systemd.unified_cgroup_hierarchy=0`` to the boot options as well.
+
 You have now configured the system to use cgroups. To create
 the actual cgroups that DOMjudge will use you need to run::
 

judge/create_cgroups.in

@@ -19,23 +19,30 @@ cgroup_error_and_usage () {
 }
 
 # Check whether cgroup v2 is available.
+cgroupv2_available=''
 fs_type=$(awk '$2 == "/sys/fs/cgroup" {print $3}' /proc/mounts)
 if [ "$fs_type" = "cgroup2" ]; then
     kernel_version=$(uname -r)
     major=$(echo "$kernel_version" | cut -d '.' -f 1)
     minor=$(echo "$kernel_version" | cut -d '.' -f 2)
     if [ "$major" -lt 5 ] || { [ "$major" -eq 5 ] && [ "$minor" -lt 19 ]; }; then
-        echo "WARNING: Kernel ($kernel_version) is too old to record peak RAM usage" >&2
+        echo "Kernel ($kernel_version) is too old to record peak RAM usage with cgroup V2, trying cgroup V1" >&2
+    else
+        cgroupv2_available=1
     fi
+fi
+
+if [ -n "$cgroupv2_available" ]; 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
 
+# Trying cgroup V1:
+
 for i in cpuset memory; do
     mkdir -p $CGROUPBASE/$i
     if [ ! -d $CGROUPBASE/$i/ ]; then