sched: Force the address order of each sched class descriptor

In order to make a micro optimization in pick_next_task(), the order of the
sched class descriptor address must be in the same order as their priority
to each other. That is:

 &idle_sched_class < &fair_sched_class < &rt_sched_class <
 &dl_sched_class < &stop_sched_class

In order to guarantee this order of the sched class descriptors, add each
one into their own data section and force the order in the linker script.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/157675913272.349305.8936736338884044103.stgit@localhost.localdomain

kernel/sched/deadline.c

@@ -2479,7 +2479,8 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 	}
 }
 
-const struct sched_class dl_sched_class = {
+const struct sched_class dl_sched_class
+	__attribute__((section("__dl_sched_class"))) = {
 	.next			= &rt_sched_class,
 	.enqueue_task		= enqueue_task_dl,
 	.dequeue_task		= dequeue_task_dl,

kernel/sched/fair.c

@@ -11122,7 +11122,8 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
 /*
  * All the scheduling class methods:
  */
-const struct sched_class fair_sched_class = {
+const struct sched_class fair_sched_class
+	__attribute__((section("__fair_sched_class"))) = {
 	.next			= &idle_sched_class,
 	.enqueue_task		= enqueue_task_fair,
 	.dequeue_task		= dequeue_task_fair,

kernel/sched/idle.c

@@ -453,7 +453,8 @@ static void update_curr_idle(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
-const struct sched_class idle_sched_class = {
+const struct sched_class idle_sched_class
+	__attribute__((section("__idle_sched_class"))) = {
 	/* .next is NULL */
 	/* no enqueue/yield_task for idle tasks */
 

kernel/sched/rt.c

@@ -2429,7 +2429,8 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
 		return 0;
 }
 
-const struct sched_class rt_sched_class = {
+const struct sched_class rt_sched_class
+	__attribute__((section("__rt_sched_class"))) = {
 	.next			= &fair_sched_class,
 	.enqueue_task		= enqueue_task_rt,
 	.dequeue_task		= dequeue_task_rt,

kernel/sched/stop_task.c

@@ -109,7 +109,8 @@ static void update_curr_stop(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU stop tasks:
  */
-const struct sched_class stop_sched_class = {
+const struct sched_class stop_sched_class
+	__attribute__((section("__stop_sched_class"))) = {
 	.next			= &dl_sched_class,
 
 	.enqueue_task		= enqueue_task_stop,