hooks: Add cset CPU isolation hook

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

libvirt_hooks/hooks/cset.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+
+#
+# Original author: Rokas Kupstys <rokups@zoho.com>
+# Heavily modified by: Danny Lin <danny@kdrag0n.dev>
+#
+# This hook uses the `cset` tool to dynamically isolate and unisolate CPUs using
+# the kernel's cgroup cpusets feature. While it's not as effective as
+# full kernel-level scheduler and timekeeping isolation, it still does wonders
+# for VM latency as compared to not isolating CPUs at all. Note that vCPU thread
+# affinity is a must for this to work properly.
+#
+# Original source: https://rokups.github.io/#!pages/gaming-vm-performance.md
+#
+# Target file locations:
+#   - $SYSCONFDIR/hooks/qemu.d/vm_name/prepare/begin/cset.sh
+#   - $SYSCONFDIR/hooks/qemu.d/vm_name/release/end/cset.sh
+# $SYSCONFDIR is usually /etc/libvirt.
+#
+
+TOTAL_CORES='0-7'
+TOTAL_CORES_MASK=FF             # 0-7, bitmask 0b11111111
+HOST_CORES='0,4'                # Cores reserved for host
+HOST_CORES_MASK=11              # 0,4, bitmask 0b00010001
+VIRT_CORES='1-3,5-7'            # Cores reserved for virtual machine(s)
+
+VM_NAME="$1"
+VM_ACTION="$2/$3"
+
+function shield_vm() {
+    cset -m set -c $TOTAL_CORES -s machine.slice
+    cset -m shield --kthread on --cpu $VIRT_CORES
+}
+
+function unshield_vm() {
+    cset -m shield --reset
+}
+
+# For convenient manual invocation
+if [[ "$VM_NAME" == "shield" ]]; then
+    shield_vm
+    exit
+elif [[ "$VM_NAME" == "unshield" ]]; then
+    unshield_vm
+    exit
+fi
+
+if [[ "$VM_ACTION" == "prepare/begin" ]]; then
+    echo "libvirt-qemu cset: Reserving CPUs $VIRT_CORES for VM $VM_NAME" > /dev/kmsg 2>&1
+    shield_vm > /dev/kmsg 2>&1
+
+    # the kernel's dirty page writeback mechanism uses kthread workers. They introduce
+    # massive arbitrary latencies when doing disk writes on the host and aren't
+    # migrated by cset. Restrict the workqueue to use only cpu 0.
+    echo $HOST_CORES_MASK > /sys/bus/workqueue/devices/writeback/cpumask
+    echo 0 > /sys/bus/workqueue/devices/writeback/numa
+
+    echo "libvirt-qemu cset: Successfully reserved CPUs $VIRT_CORES" > /dev/kmsg 2>&1
+elif [[ "$VM_ACTION" == "release/end" ]]; then
+    echo "libvirt-qemu cset: Releasing CPUs $VIRT_CORES from VM $VM_NAME" > /dev/kmsg 2>&1
+    unshield_vm > /dev/kmsg 2>&1
+
+    # Revert changes made to the writeback workqueue
+    echo $TOTAL_CORES_MASK > /sys/bus/workqueue/devices/writeback/cpumask
+    echo 1 > /sys/bus/workqueue/devices/writeback/numa
+
+    echo "libvirt-qemu cset: Successfully released CPUs $VIRT_CORES" > /dev/kmsg 2>&1
+fi