sched: Simplify sys_sched_rr_get_interval() system call

By removing the need for it to know details of scheduling classes.

This allows PlugSched to define orthogonal scheduling classes.

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <06d1b89ee15a0eef82d7.1253496713@mudlark.pw.nest>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_idletask.c

@@ -97,6 +97,11 @@ static void prio_changed_idle(struct rq *rq, struct task_struct *p,
 		check_preempt_curr(rq, p, 0);
 }
 
+unsigned int get_rr_interval_idle(struct task_struct *task)
+{
+	return 0;
+}
+
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
@@ -122,6 +127,8 @@ static const struct sched_class idle_sched_class = {
 	.set_curr_task          = set_curr_task_idle,
 	.task_tick		= task_tick_idle,
 
+	.get_rr_interval	= get_rr_interval_idle,
+
 	.prio_changed		= prio_changed_idle,
 	.switched_to		= switched_to_idle,