sched_ext: Simplify UP support by enabling sched_class->balance() in UP

On SMP, SCX performs dispatch from sched_class->balance(). As balance() was
not available in UP, it instead called the internal balance function from
put_prev_task_scx() and pick_next_task_scx() to emulate the effect, which is
rather nasty.

Enabling sched_class->balance() on UP shouldn't cause any meaningful
overhead. Enable balance() on UP and drop the ugly workaround.
Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: David Vernet <void@manifault.com>

kernel/sched/core.c

@@ -5857,7 +5857,6 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
 static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
 				  struct rq_flags *rf)
 {
-#ifdef CONFIG_SMP
 	const struct sched_class *start_class = prev->sched_class;
 	const struct sched_class *class;
 
@@ -5880,10 +5879,9 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
 	 * a runnable task of @class priority or higher.
 	 */
 	for_active_class_range(class, start_class, &idle_sched_class) {
-		if (class->balance(rq, prev, rf))
+		if (class->balance && class->balance(rq, prev, rf))
 			break;
 	}
-#endif
 
 	put_prev_task(rq, prev);
 

kernel/sched/ext.c

@@ -2616,7 +2616,6 @@ static int balance_one(struct rq *rq, struct task_struct *prev, bool local)
 	return has_tasks;
 }
 
-#ifdef CONFIG_SMP
 static int balance_scx(struct rq *rq, struct task_struct *prev,
 		       struct rq_flags *rf)
 {
@@ -2650,7 +2649,6 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
 
 	return ret;
 }
-#endif
 
 static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 {
@@ -2719,37 +2717,6 @@ static void process_ddsp_deferred_locals(struct rq *rq)
 
 static void put_prev_task_scx(struct rq *rq, struct task_struct *p)
 {
-#ifndef CONFIG_SMP
-	/*
-	 * UP workaround.
-	 *
-	 * Because SCX may transfer tasks across CPUs during dispatch, dispatch
-	 * is performed from its balance operation which isn't called in UP.
-	 * Let's work around by calling it from the operations which come right
-	 * after.
-	 *
-	 * 1. If the prev task is on SCX, pick_next_task() calls
-	 *    .put_prev_task() right after. As .put_prev_task() is also called
-	 *    from other places, we need to distinguish the calls which can be
-	 *    done by looking at the previous task's state - if still queued or
-	 *    dequeued with %SCX_DEQ_SLEEP, the caller must be pick_next_task().
-	 *    This case is handled here.
-	 *
-	 * 2. If the prev task is not on SCX, the first following call into SCX
-	 *    will be .pick_next_task(), which is covered by calling
-	 *    balance_scx() from pick_next_task_scx().
-	 *
-	 * Note that we can't merge the first case into the second as
-	 * balance_scx() must be called before the previous SCX task goes
-	 * through put_prev_task_scx().
-	 *
-         * @rq is pinned and can't be unlocked. As UP doesn't transfer tasks
-         * around, balance_one() doesn't need to.
-	 */
-	if (p->scx.flags & (SCX_TASK_QUEUED | SCX_TASK_DEQD_FOR_SLEEP))
-		balance_one(rq, p, true);
-#endif
-
 	update_curr_scx(rq);
 
 	/* see dequeue_task_scx() on why we skip when !QUEUED */
@@ -2807,12 +2774,6 @@ static struct task_struct *pick_next_task_scx(struct rq *rq)
 {
 	struct task_struct *p;
 
-#ifndef CONFIG_SMP
-	/* UP workaround - see the comment at the head of put_prev_task_scx() */
-	if (unlikely(rq->curr->sched_class != &ext_sched_class))
-		balance_one(rq, rq->curr, true);
-#endif
-
 	p = first_local_task(rq);
 	if (!p)
 		return NULL;
@@ -3673,6 +3634,7 @@ DEFINE_SCHED_CLASS(ext) = {
 
 	.wakeup_preempt		= wakeup_preempt_scx,
 
+	.balance		= balance_scx,
 	.pick_next_task		= pick_next_task_scx,
 
 	.put_prev_task		= put_prev_task_scx,
@@ -3681,7 +3643,6 @@ DEFINE_SCHED_CLASS(ext) = {
 	.switch_class		= switch_class_scx,
 
 #ifdef CONFIG_SMP
-	.balance		= balance_scx,
 	.select_task_rq		= select_task_rq_scx,
 	.task_woken		= task_woken_scx,
 	.set_cpus_allowed	= set_cpus_allowed_scx,

kernel/sched/sched.h

@@ -2361,6 +2361,7 @@ struct sched_class {
 
 	void (*wakeup_preempt)(struct rq *rq, struct task_struct *p, int flags);
 
+	int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
 	struct task_struct *(*pick_next_task)(struct rq *rq);
 
 	void (*put_prev_task)(struct rq *rq, struct task_struct *p);
@@ -2369,7 +2370,6 @@ struct sched_class {
 	void (*switch_class)(struct rq *rq, struct task_struct *next);
 
 #ifdef CONFIG_SMP
-	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);