genirq: Set irq thread to RT priority on creation

When a threaded irq handler is installed the irq thread is initially
created on normal scheduling priority. Only after the irq thread is
woken up it sets its priority to RT_FIFO MAX_USER_RT_PRIO/2 itself.

This means that interrupts that occur directly after the irq handler
is installed will be handled on a normal scheduling priority instead
of the realtime priority that one would expect.

Fix this by setting the RT priority on creation of the irq_thread.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Cc: Sebastian Andrzej Siewior  <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1370254322-17240-1-git-send-email-meltedpianoman@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/manage.c

@@ -840,9 +840,6 @@ static void irq_thread_dtor(struct callback_head *unused)
 static int irq_thread(void *data)
 {
 	struct callback_head on_exit_work;
-	static const struct sched_param param = {
-		.sched_priority = MAX_USER_RT_PRIO/2,
-	};
 	struct irqaction *action = data;
 	struct irq_desc *desc = irq_to_desc(action->irq);
 	irqreturn_t (*handler_fn)(struct irq_desc *desc,
@@ -854,8 +851,6 @@ static int irq_thread(void *data)
 	else
 		handler_fn = irq_thread_fn;
 
-	sched_setscheduler(current, SCHED_FIFO, &param);
-
 	init_task_work(&on_exit_work, irq_thread_dtor);
 	task_work_add(current, &on_exit_work, false);
 
@@ -950,6 +945,9 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 	 */
 	if (new->thread_fn && !nested) {
 		struct task_struct *t;
+		static const struct sched_param param = {
+			.sched_priority = MAX_USER_RT_PRIO/2,
+		};
 
 		t = kthread_create(irq_thread, new, "irq/%d-%s", irq,
 				   new->name);
@@ -957,6 +955,9 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 			ret = PTR_ERR(t);
 			goto out_mput;
 		}
+
+		sched_setscheduler(t, SCHED_FIFO, &param);
+
 		/*
 		 * We keep the reference to the task struct even if
 		 * the thread dies to avoid that the interrupt code