hooks: Add CFS nice level adjustment hook

Signed-off-by: Danny Lin <danny@kdrag0n.dev>

libvirt_hooks/hooks/nice.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+
+#
+# Author: Danny Lin <danny@kdrag0n.dev>
+#
+# This hook sets the CFS "nice level" of the vCPU threads to the configured
+# value. This can be useful as a replacement for using the real-time FIFO
+# scheduling policy (SCHED_FIFO) through libvirt since it causes lockups on
+# some systems. It can improve VM responsiveness when CPU load on the host is
+# high by making the CFS scheduler prioritize runnable vCPU threads over other
+# miscellaneous processes which are "nicer".
+#
+# Note that this is primarily intended for setups where QEMU is not running as
+# its own dedicated user as otherwise it is preferred to use udev rules to
+# set the user's default nice level instead. Some setups necessitate running
+# QEMU as another user for PulseAudio or other reasons, which is where this
+# script is useful.
+#
+# Target file location: $SYSCONFDIR/hooks/qemu.d/vm_name/started/begin/nice.sh
+# $SYSCONFDIR is usually /etc/libvirt.
+#
+
+# Ranges from 20 (lowest priority) to -20 (highest priority)
+TARGET_NICE="-1"
+
+VM_NAME="$1"
+
+# Set the nice of all vCPUs
+for grp in /sys/fs/cgroup/cpu/machine.slice/machine-qemu*$VM_NAME.scope/vcpu*
+do
+    echo "libvirt-qemu nice: Setting $(basename $grp)'s nice level to $TARGET_NICE" > /dev/kmsg
+    for pid in $(cat $grp/tasks)
+    do
+        renice -n "$TARGET_NICE" -p "$pid" 2> /dev/null
+    done
+done
+
+echo "libvirt-qemu nice: Prioritized vCPU threads of VM '$VM_NAME'" > /dev/kmsg