sched: Remove struct sched_class::next field

Now that the sched_class descriptors are defined in order via the linker
script vmlinux.lds.h, there's no reason to have a "next" pointer to the
previous priroity structure. The order of the sturctures can be aligned as
an array, and used to index and find the next sched_class descriptor.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191219214558.845353593@goodmis.org

kernel/sched/deadline.c

@@ -2481,7 +2481,6 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 
 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,
 	.yield_task		= yield_task_dl,

kernel/sched/fair.c

@@ -11124,7 +11124,6 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
  */
 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,
 	.yield_task		= yield_task_fair,

kernel/sched/idle.c

@@ -455,7 +455,6 @@ static void update_curr_idle(struct rq *rq)
  */
 const struct sched_class idle_sched_class
 	__attribute__((section("__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

@@ -2431,7 +2431,6 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
 
 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,
 	.yield_task		= yield_task_rt,

kernel/sched/sched.h

@@ -1754,7 +1754,6 @@ extern const u32		sched_prio_to_wmult[40];
 #define RETRY_TASK		((void *)-1UL)
 
 struct sched_class {
-	const struct sched_class *next;
 
 #ifdef CONFIG_UCLAMP_TASK
 	int uclamp_enabled;

kernel/sched/stop_task.c

@@ -111,7 +111,6 @@ static void update_curr_stop(struct rq *rq)
  */
 const struct sched_class stop_sched_class
 	__attribute__((section("__stop_sched_class"))) = {
-	.next			= &dl_sched_class,
 
 	.enqueue_task		= enqueue_task_stop,
 	.dequeue_task		= dequeue_task_stop,