misc: Add an example for policy script

Add a do-nothing policy script as an example, pre-implement some basic
logic that could be used for applying affinity_hinting setting strictly
for IRQs of certain drivers.

misc/irqbalance.policy.d/follow-affinity-hint.example

@@ -0,0 +1,39 @@
+#!/bin/sh
+# Do not edit this file, create your own policy script and make it
+# executable for irqbalance process, you can use this file as an
+# boilerplate.
+
+SYS_DEV_PATH=$1
+IRQ_NUM=$2
+
+IRQ_PATH=/proc/irq/$IRQ_NUM
+UEVENT_FILE=$SYS_DEV_PATH/uevent
+
+# Scripts below is an example for banning certain IRQs from
+# irqbalance and strictly apply their affinity_hint setting
+[[ ! -e $UEVENT_FILE ]] && exit 1
+
+# IRQs from following drivers will be handled by this script
+# Driver names should be separated by space
+AFFINITY_WHITELIST=""
+
+while read line; do
+    if [[ $line == "DRIVER="* ]] && \
+        [[ " $AFFINITY_WHITELIST " == *" ${line#DRIVER=} "* ]]; then
+        affinity_hint=$(cat $IRQ_PATH/affinity_hint 2>/dev/null)
+        # Check if affinity_hint value have at least one bit set
+        if [[ ! "$affinity_hint" =~ ^[0,]*$ ]]; then
+            # Ban it from irqbalance so it won't get balanced,
+			# we'll follow its affinity_hint setting
+            echo "ban=true"
+            # If the affinity_hint value is valid, kernel would set
+            # the same value for smp_affinity. But force to set that
+            # again in case the IRQ was balanced before.
+            echo "$affinity_hint" > $IRQ_PATH/smp_affinity
+            # Stop further script processing
+            exit 0
+        fi
+    fi
+done <<< "$(cat $UEVENT_FILE)"
+
+exit 1