sched: mark scheduling classes as const

mark scheduling classes as const. The speeds up the code
a bit and shrinks it:

   text    data     bss     dec     hex filename
  40027    4018     292   44337    ad31 sched.o.before
  40190    3842     292   44324    ad24 sched.o.after
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/sched.h

@@ -863,7 +863,7 @@ struct rq;
 struct sched_domain;
 
 struct sched_class {
-	struct sched_class *next;
+	const struct sched_class *next;
 
 	void (*enqueue_task) (struct rq *rq, struct task_struct *p, int wakeup);
 	void (*dequeue_task) (struct rq *rq, struct task_struct *p, int sleep);
@@ -949,7 +949,7 @@ struct task_struct {
 
 	int prio, static_prio, normal_prio;
 	struct list_head run_list;
-	struct sched_class *sched_class;
+	const struct sched_class *sched_class;
 	struct sched_entity se;
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS

kernel/sched.c

@@ -847,9 +847,9 @@ static int balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
 		      int *this_best_prio, struct rq_iterator *iterator);
 
 #include "sched_stats.h"
-#include "sched_rt.c"
-#include "sched_fair.c"
 #include "sched_idletask.c"
+#include "sched_fair.c"
+#include "sched_rt.c"
 #ifdef CONFIG_SCHED_DEBUG
 # include "sched_debug.c"
 #endif
@@ -2251,7 +2251,7 @@ static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
 		      struct sched_domain *sd, enum cpu_idle_type idle,
 		      int *all_pinned)
 {
-	struct sched_class *class = sched_class_highest;
+	const struct sched_class *class = sched_class_highest;
 	unsigned long total_load_moved = 0;
 	int this_best_prio = this_rq->curr->prio;
 
@@ -2276,7 +2276,7 @@ static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
 static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
 			 struct sched_domain *sd, enum cpu_idle_type idle)
 {
-	struct sched_class *class;
+	const struct sched_class *class;
 	int this_best_prio = MAX_PRIO;
 
 	for (class = sched_class_highest; class; class = class->next)
@@ -3432,7 +3432,7 @@ static inline void schedule_debug(struct task_struct *prev)
 static inline struct task_struct *
 pick_next_task(struct rq *rq, struct task_struct *prev)
 {
-	struct sched_class *class;
+	const struct sched_class *class;
 	struct task_struct *p;
 
 	/*
@@ -6504,13 +6504,6 @@ void __init sched_init(void)
 	int highest_cpu = 0;
 	int i, j;
 
-	/*
-	 * Link up the scheduling class hierarchy:
-	 */
-	rt_sched_class.next = &fair_sched_class;
-	fair_sched_class.next = &idle_sched_class;
-	idle_sched_class.next = NULL;
-
 	for_each_possible_cpu(i) {
 		struct rt_prio_array *array;
 		struct rq *rq;

kernel/sched_fair.c

@@ -76,8 +76,6 @@ const_debug unsigned int sysctl_sched_batch_wakeup_granularity = 25000000UL;
  */
 const_debug unsigned int sysctl_sched_wakeup_granularity = 2000000UL;
 
-extern struct sched_class fair_sched_class;
-
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
@@ -1031,7 +1029,8 @@ static void set_curr_task_fair(struct rq *rq)
 /*
  * All the scheduling class methods:
  */
-struct sched_class fair_sched_class __read_mostly = {
+static const struct sched_class fair_sched_class = {
+	.next			= &idle_sched_class,
 	.enqueue_task		= enqueue_task_fair,
 	.dequeue_task		= dequeue_task_fair,
 	.yield_task		= yield_task_fair,

kernel/sched_idletask.c

@@ -57,7 +57,8 @@ static void set_curr_task_idle(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
-static struct sched_class idle_sched_class __read_mostly = {
+const struct sched_class idle_sched_class = {
+	/* .next is NULL */
 	/* no enqueue/yield_task for idle tasks */
 
 	/* dequeue is not valid, we print a debug message there: */

kernel/sched_rt.c

@@ -225,7 +225,8 @@ static void set_curr_task_rt(struct rq *rq)
 	p->se.exec_start = rq->clock;
 }
 
-static struct sched_class rt_sched_class __read_mostly = {
+const struct sched_class rt_sched_class = {
+	.next			= &fair_sched_class,
 	.enqueue_task		= enqueue_task_rt,
 	.dequeue_task		= dequeue_task_rt,
 	.yield_task		= yield_task_rt,