Improve documentation and logging for banned cpus

This patch have no functional modification. Just improve the doc and log
for isolcpu, nohz_full and numa node banning cpus, for providing more
info for users.

Signed-off-by: Tao Liu <ltao@redhat.com>

cputree.c

@@ -159,14 +159,14 @@ static void setup_banned_cpus(void)
 	cpus_or(banned_cpus, nohz_full, isolated_cpus);
 
 	cpumask_scnprintf(buffer, 4096, isolated_cpus);
-	log(TO_CONSOLE, LOG_INFO, "Isolated CPUs: %s\n", buffer);
+	log(TO_CONSOLE, LOG_INFO, "Prevent irq assignment to these isolated CPUs: %s\n", buffer);
 	cpumask_scnprintf(buffer, 4096, nohz_full);
-	log(TO_CONSOLE, LOG_INFO, "Adaptive-ticks CPUs: %s\n", buffer);
+	log(TO_CONSOLE, LOG_INFO, "Prevent irq assignment to these adaptive-ticks CPUs: %s\n", buffer);
 out:
 #ifdef HAVE_THERMAL
 	cpus_or(banned_cpus, banned_cpus, thermal_banned_cpus);
 	cpumask_scnprintf(buffer, 4096, thermal_banned_cpus);
-	log(TO_CONSOLE, LOG_INFO, "Thermal-banned CPUs: %s\n", buffer);
+	log(TO_CONSOLE, LOG_INFO, "Prevent irq assignment to these thermal-banned CPUs: %s\n", buffer);
 #endif
 	cpumask_scnprintf(buffer, 4096, banned_cpus);
 	log(TO_CONSOLE, LOG_INFO, "Banned CPUs: %s\n", buffer);

irqbalance.1

@@ -167,7 +167,7 @@ Same as --debug.
 .B IRQBALANCE_BANNED_CPUS
 Provides a mask of CPUs which irqbalance should ignore and never assign interrupts to.
 If not specified, irqbalance use mask of isolated and adaptive-ticks CPUs on the
-system as the default value.
+system as the default value. The "isolcpus=" boot parameter specifies the isolated CPUs. The "nohz_full=" boot parameter specifies the adaptive-ticks CPUs. By default, no CPU will be an isolated or adaptive-ticks CPU.
 This is a hexmask without the leading ’0x’. On systems with large numbers of
 processors, each group of eight hex digits is separated by a comma ’,’. i.e.
 ‘export IRQBALANCE_BANNED_CPUS=fc0‘ would prevent irqbalance from assigning irqs

placement.c

@@ -135,8 +135,12 @@ static void place_irq_in_node(struct irq_info *info, void *data __attribute__((u
 		 * Need to make sure this node is elligible for migration
 		 * given the banned cpu list
 		 */
-		if (!cpus_intersects(irq_numa_node(info)->mask, unbanned_cpus))
+		if (!cpus_intersects(irq_numa_node(info)->mask, unbanned_cpus)) {
+			log(TO_CONSOLE, LOG_WARNING, "There is no suitable CPU in node:%d.\n", irq_numa_node(info)->number);
+			log(TO_CONSOLE, LOG_WARNING, "Irqbalance dispatch irq:%d to other node.\n", info->irq);
 			goto find_placement;
+		}
+
 		/*
 		 * This irq belongs to a device with a preferred numa node
 		 * put it on that node