sched: Rework pick_next_task()

The current rule is that:

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

And many classes implement it directly as such. Change things around
to make pick_next_task() optional while also changing the definition to:

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

The reason is that sched_ext would like to have a 'final' call that
knows the next task. By placing put_prev_task() right next to
set_next_task() (as it already is for sched_core) this becomes
trivial.

As a bonus, this is a nice cleanup on its own.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240813224016.051225657@infradead.org

kernel/sched/core.c

@@ -5893,8 +5893,9 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 
 		/* Assume the next prioritized class is idle_sched_class */
 		if (!p) {
+			p = pick_task_idle(rq);
 			put_prev_task(rq, prev);
-			p = pick_next_task_idle(rq);
+			set_next_task_first(rq, p);
 		}
 
 		/*
@@ -5916,12 +5917,20 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 
 restart:
 	prev_balance(rq, prev, rf);
-	put_prev_task(rq, prev);
 
 	for_each_class(class) {
-		p = class->pick_next_task(rq);
-		if (p)
-			return p;
+		if (class->pick_next_task) {
+			p = class->pick_next_task(rq, prev);
+			if (p)
+				return p;
+		} else {
+			p = class->pick_task(rq);
+			if (p) {
+				put_prev_task(rq, prev);
+				set_next_task_first(rq, p);
+				return p;
+			}
+		}
 	}
 
 	BUG(); /* The idle class should always have a runnable task. */
@@ -6017,7 +6026,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	}
 
 	prev_balance(rq, prev, rf);
-	put_prev_task(rq, prev);
 
 	smt_mask = cpu_smt_mask(cpu);
 	need_sync = !!rq->core->core_cookie;
@@ -6184,6 +6192,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	}
 
 out_set_next:
+	put_prev_task(rq, prev);
 	set_next_task_first(rq, next);
 out:
 	if (rq->core->core_forceidle_count && next == rq->idle)

kernel/sched/deadline.c

@@ -2431,28 +2431,10 @@ static struct task_struct *__pick_task_dl(struct rq *rq)
 	return p;
 }
 
-#ifdef CONFIG_SMP
 static struct task_struct *pick_task_dl(struct rq *rq)
 {
 	return __pick_task_dl(rq);
 }
-#endif
-
-static struct task_struct *pick_next_task_dl(struct rq *rq)
-{
-	struct task_struct *p;
-
-	p = __pick_task_dl(rq);
-	if (!p)
-		return p;
-
-	if (p->dl_server)
-		p->sched_class->set_next_task(rq, p, true);
-	else
-		set_next_task_dl(rq, p, true);
-
-	return p;
-}
 
 static void put_prev_task_dl(struct rq *rq, struct task_struct *p)
 {
@@ -3146,13 +3128,12 @@ DEFINE_SCHED_CLASS(dl) = {
 
 	.wakeup_preempt		= wakeup_preempt_dl,
 
-	.pick_next_task		= pick_next_task_dl,
+	.pick_task		= pick_task_dl,
 	.put_prev_task		= put_prev_task_dl,
 	.set_next_task		= set_next_task_dl,
 
 #ifdef CONFIG_SMP
 	.balance		= balance_dl,
-	.pick_task		= pick_task_dl,
 	.select_task_rq		= select_task_rq_dl,
 	.migrate_task_rq	= migrate_task_rq_dl,
 	.set_cpus_allowed       = set_cpus_allowed_dl,

kernel/sched/fair.c

@@ -8777,7 +8777,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 	se = &p->se;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-	if (!prev || prev->sched_class != &fair_sched_class)
+	if (prev->sched_class != &fair_sched_class)
 		goto simple;
 
 	/*
@@ -8819,8 +8819,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 
 simple:
 #endif
-	if (prev)
-		put_prev_task(rq, prev);
+	put_prev_task(rq, prev);
 	set_next_task_fair(rq, p, true);
 	return p;
 
@@ -8850,9 +8849,9 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 	return NULL;
 }
 
-static struct task_struct *__pick_next_task_fair(struct rq *rq)
+static struct task_struct *__pick_next_task_fair(struct rq *rq, struct task_struct *prev)
 {
-	return pick_next_task_fair(rq, NULL, NULL);
+	return pick_next_task_fair(rq, prev, NULL);
 }
 
 static bool fair_server_has_tasks(struct sched_dl_entity *dl_se)
@@ -13490,13 +13489,13 @@ DEFINE_SCHED_CLASS(fair) = {
 
 	.wakeup_preempt		= check_preempt_wakeup_fair,
 
+	.pick_task		= pick_task_fair,
 	.pick_next_task		= __pick_next_task_fair,
 	.put_prev_task		= put_prev_task_fair,
 	.set_next_task          = set_next_task_fair,
 
 #ifdef CONFIG_SMP
 	.balance		= balance_fair,
-	.pick_task		= pick_task_fair,
 	.select_task_rq		= select_task_rq_fair,
 	.migrate_task_rq	= migrate_task_rq_fair,
 

kernel/sched/idle.c

@@ -462,21 +462,10 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
 	next->se.exec_start = rq_clock_task(rq);
 }
 
-#ifdef CONFIG_SMP
-static struct task_struct *pick_task_idle(struct rq *rq)
+struct task_struct *pick_task_idle(struct rq *rq)
 {
 	return rq->idle;
 }
-#endif
-
-struct task_struct *pick_next_task_idle(struct rq *rq)
-{
-	struct task_struct *next = rq->idle;
-
-	set_next_task_idle(rq, next, true);
-
-	return next;
-}
 
 /*
  * It is not legal to sleep in the idle task - print a warning
@@ -531,13 +520,12 @@ DEFINE_SCHED_CLASS(idle) = {
 
 	.wakeup_preempt		= wakeup_preempt_idle,
 
-	.pick_next_task		= pick_next_task_idle,
+	.pick_task		= pick_task_idle,
 	.put_prev_task		= put_prev_task_idle,
 	.set_next_task          = set_next_task_idle,
 
 #ifdef CONFIG_SMP
 	.balance		= balance_idle,
-	.pick_task		= pick_task_idle,
 	.select_task_rq		= select_task_rq_idle,
 	.set_cpus_allowed	= set_cpus_allowed_common,
 #endif

kernel/sched/rt.c

@@ -1748,16 +1748,6 @@ static struct task_struct *pick_task_rt(struct rq *rq)
 	return p;
 }
 
-static struct task_struct *pick_next_task_rt(struct rq *rq)
-{
-	struct task_struct *p = pick_task_rt(rq);
-
-	if (p)
-		set_next_task_rt(rq, p, true);
-
-	return p;
-}
-
 static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 {
 	struct sched_rt_entity *rt_se = &p->rt;
@@ -2645,13 +2635,12 @@ DEFINE_SCHED_CLASS(rt) = {
 
 	.wakeup_preempt		= wakeup_preempt_rt,
 
-	.pick_next_task		= pick_next_task_rt,
+	.pick_task		= pick_task_rt,
 	.put_prev_task		= put_prev_task_rt,
 	.set_next_task          = set_next_task_rt,
 
 #ifdef CONFIG_SMP
 	.balance		= balance_rt,
-	.pick_task		= pick_task_rt,
 	.select_task_rq		= select_task_rq_rt,
 	.set_cpus_allowed       = set_cpus_allowed_common,
 	.rq_online              = rq_online_rt,

kernel/sched/sched.h

@@ -2300,7 +2300,17 @@ struct sched_class {
 
 	void (*wakeup_preempt)(struct rq *rq, struct task_struct *p, int flags);
 
-	struct task_struct *(*pick_next_task)(struct rq *rq);
+	struct task_struct *(*pick_task)(struct rq *rq);
+	/*
+	 * Optional! When implemented pick_next_task() should be equivalent to:
+	 *
+	 *   next = pick_task();
+	 *   if (next) {
+	 *       put_prev_task(prev);
+	 *       set_next_task_first(next);
+	 *   }
+	 */
+	struct task_struct *(*pick_next_task)(struct rq *rq, struct task_struct *prev);
 
 	void (*put_prev_task)(struct rq *rq, struct task_struct *p);
 	void (*set_next_task)(struct rq *rq, struct task_struct *p, bool first);
@@ -2309,8 +2319,6 @@ struct sched_class {
 	int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
 	int  (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);
 
-	struct task_struct * (*pick_task)(struct rq *rq);
-
 	void (*migrate_task_rq)(struct task_struct *p, int new_cpu);
 
 	void (*task_woken)(struct rq *this_rq, struct task_struct *task);
@@ -2421,7 +2429,7 @@ static inline bool sched_fair_runnable(struct rq *rq)
 }
 
 extern struct task_struct *pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
-extern struct task_struct *pick_next_task_idle(struct rq *rq);
+extern struct task_struct *pick_task_idle(struct rq *rq);
 
 #define SCA_CHECK		0x01
 #define SCA_MIGRATE_DISABLE	0x02

kernel/sched/stop_task.c

@@ -41,16 +41,6 @@ static struct task_struct *pick_task_stop(struct rq *rq)
 	return rq->stop;
 }
 
-static struct task_struct *pick_next_task_stop(struct rq *rq)
-{
-	struct task_struct *p = pick_task_stop(rq);
-
-	if (p)
-		set_next_task_stop(rq, p, true);
-
-	return p;
-}
-
 static void
 enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 {
@@ -112,13 +102,12 @@ DEFINE_SCHED_CLASS(stop) = {
 
 	.wakeup_preempt		= wakeup_preempt_stop,
 
-	.pick_next_task		= pick_next_task_stop,
+	.pick_task		= pick_task_stop,
 	.put_prev_task		= put_prev_task_stop,
 	.set_next_task          = set_next_task_stop,
 
 #ifdef CONFIG_SMP
 	.balance		= balance_stop,
-	.pick_task		= pick_task_stop,
 	.select_task_rq		= select_task_rq_stop,
 	.set_cpus_allowed	= set_cpus_allowed_common,
 #endif