cpu hotplug: fix ksoftirqd termination on cpu hotplug with naughty realtime process

Fix ksoftirqd termination on cpu hotplug with naughty real time process.

Assuming the following case:

 - Try to hot remove CPU2 from CPU1.
 - There is a real time process on CPU2, and that process doesn't sleep at all.
 - That rt process and ksoftirqd/2 is migrated to the CPU0

Then ksoftirqd/2 can't stop becasue that rt process runs everlastingly on
CPU0, and CPU1 waiting the ksoftirqd/2's termination hangs up.  To fix this
problem, set the priority of ksoftirqd/2 to max one before kthread_stop().

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/softirq.c

@@ -614,12 +614,16 @@ static int __cpuinit cpu_callback(struct notifier_block *nfb,
 		kthread_bind(per_cpu(ksoftirqd, hotcpu),
 			     any_online_cpu(cpu_online_map));
 	case CPU_DEAD:
-	case CPU_DEAD_FROZEN:
+	case CPU_DEAD_FROZEN: {
+		struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
+
 		p = per_cpu(ksoftirqd, hotcpu);
 		per_cpu(ksoftirqd, hotcpu) = NULL;
+		sched_setscheduler(p, SCHED_FIFO, &param);
 		kthread_stop(p);
 		takeover_tasklets(hotcpu);
 		break;
+	}
 #endif /* CONFIG_HOTPLUG_CPU */
  	}
 	return NOTIFY_OK;