sched: Clean up DL server vs core sched

Abide by the simple rule:

  pick_next_task() := pick_task() + set_next_task(.first = true)

This allows us to trivially get rid of server_pick_next() and things
collapse nicely.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240813224015.837303391@infradead.org

include/linux/sched.h

@@ -692,7 +692,6 @@ struct sched_dl_entity {
 	 */
 	struct rq			*rq;
 	dl_server_has_tasks_f		server_has_tasks;
-	dl_server_pick_f		server_pick_next;
 	dl_server_pick_f		server_pick_task;
 
 #ifdef CONFIG_RT_MUTEXES

kernel/sched/deadline.c

@@ -1665,12 +1665,10 @@ void dl_server_stop(struct sched_dl_entity *dl_se)
 
 void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_has_tasks_f has_tasks,
-		    dl_server_pick_f pick_next,
 		    dl_server_pick_f pick_task)
 {
 	dl_se->rq = rq;
 	dl_se->server_has_tasks = has_tasks;
-	dl_se->server_pick_next = pick_next;
 	dl_se->server_pick_task = pick_task;
 }
 
@@ -2404,9 +2402,8 @@ static struct sched_dl_entity *pick_next_dl_entity(struct dl_rq *dl_rq)
 /*
  * __pick_next_task_dl - Helper to pick the next -deadline task to run.
  * @rq: The runqueue to pick the next task from.
- * @peek: If true, just peek at the next task. Only relevant for dlserver.
  */
-static struct task_struct *__pick_next_task_dl(struct rq *rq, bool peek)
+static struct task_struct *__pick_task_dl(struct rq *rq)
 {
 	struct sched_dl_entity *dl_se;
 	struct dl_rq *dl_rq = &rq->dl;
@@ -2420,10 +2417,7 @@ static struct task_struct *__pick_next_task_dl(struct rq *rq, bool peek)
 	WARN_ON_ONCE(!dl_se);
 
 	if (dl_server(dl_se)) {
-		if (IS_ENABLED(CONFIG_SMP) && peek)
-			p = dl_se->server_pick_task(dl_se);
-		else
-			p = dl_se->server_pick_next(dl_se);
+		p = dl_se->server_pick_task(dl_se);
 		if (!p) {
 			dl_se->dl_yielded = 1;
 			update_curr_dl_se(rq, dl_se, 0);
@@ -2440,7 +2434,7 @@ static struct task_struct *__pick_next_task_dl(struct rq *rq, bool peek)
 #ifdef CONFIG_SMP
 static struct task_struct *pick_task_dl(struct rq *rq)
 {
-	return __pick_next_task_dl(rq, true);
+	return __pick_task_dl(rq);
 }
 #endif
 
@@ -2448,11 +2442,13 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
 {
 	struct task_struct *p;
 
-	p = __pick_next_task_dl(rq, false);
+	p = __pick_task_dl(rq);
 	if (!p)
 		return p;
 
-	if (!p->dl_server)
+	if (p->dl_server)
+		p->sched_class->set_next_task(rq, p, true);
+	else
 		set_next_task_dl(rq, p, true);
 
 	return p;

kernel/sched/fair.c

@@ -8862,16 +8862,7 @@ static bool fair_server_has_tasks(struct sched_dl_entity *dl_se)
 
 static struct task_struct *fair_server_pick_task(struct sched_dl_entity *dl_se)
 {
-#ifdef CONFIG_SMP
 	return pick_task_fair(dl_se->rq);
-#else
-	return NULL;
-#endif
-}
-
-static struct task_struct *fair_server_pick_next(struct sched_dl_entity *dl_se)
-{
-	return pick_next_task_fair(dl_se->rq, NULL, NULL);
 }
 
 void fair_server_init(struct rq *rq)
@@ -8880,9 +8871,7 @@ void fair_server_init(struct rq *rq)
 
 	init_dl_entity(dl_se);
 
-	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick_next,
-		       fair_server_pick_task);
-
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick_task);
 }
 
 /*

kernel/sched/sched.h

@@ -362,7 +362,6 @@ extern void dl_server_start(struct sched_dl_entity *dl_se);
 extern void dl_server_stop(struct sched_dl_entity *dl_se);
 extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_has_tasks_f has_tasks,
-		    dl_server_pick_f pick_next,
 		    dl_server_pick_f pick_task);
 
 extern void dl_server_update_idle_time(struct rq *rq,