sched: Fix delayed_dequeue vs switched_from_fair()

Commit 2e0199df25 ("sched/fair: Prepare exit/cleanup paths for delayed_dequeue")
and its follow up fixes try to deal with a rather unfortunate
situation where is task is enqueued in a new class, even though it
shouldn't have been. Mostly because the existing ->switched_to/from()
hooks are in the wrong place for this case.

This all led to Paul being able to trigger failures at something like
once per 10k CPU hours of RCU torture.

For now, do the ugly thing and move the code to the right place by
ignoring the switch hooks.

Note: Clean up the whole sched_class::switch*_{to,from}() thing.

Fixes: 2e0199df25 ("sched/fair: Prepare exit/cleanup paths for delayed_dequeue")
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241003185037.GA5594@noisy.programming.kicks-ass.net

kernel/sched/core.c

@@ -7010,20 +7010,20 @@ int default_wake_function(wait_queue_entry_t *curr, unsigned mode, int wake_flag
 }
 EXPORT_SYMBOL(default_wake_function);
 
-void __setscheduler_prio(struct task_struct *p, int prio)
+const struct sched_class *__setscheduler_class(struct task_struct *p, int prio)
 {
 	if (dl_prio(prio))
-		p->sched_class = &dl_sched_class;
-	else if (rt_prio(prio))
-		p->sched_class = &rt_sched_class;
-#ifdef CONFIG_SCHED_CLASS_EXT
-	else if (task_should_scx(p))
-		p->sched_class = &ext_sched_class;
-#endif
-	else
-		p->sched_class = &fair_sched_class;
+		return &dl_sched_class;
 
-	p->prio = prio;
+	if (rt_prio(prio))
+		return &rt_sched_class;
+
+#ifdef CONFIG_SCHED_CLASS_EXT
+	if (task_should_scx(p))
+		return &ext_sched_class;
+#endif
+
+	return &fair_sched_class;
 }
 
 #ifdef CONFIG_RT_MUTEXES
@@ -7069,7 +7069,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 {
 	int prio, oldprio, queued, running, queue_flag =
 		DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
-	const struct sched_class *prev_class;
+	const struct sched_class *prev_class, *next_class;
 	struct rq_flags rf;
 	struct rq *rq;
 
@@ -7127,6 +7127,11 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 		queue_flag &= ~DEQUEUE_MOVE;
 
 	prev_class = p->sched_class;
+	next_class = __setscheduler_class(p, prio);
+
+	if (prev_class != next_class && p->se.sched_delayed)
+		dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
+
 	queued = task_on_rq_queued(p);
 	running = task_current(rq, p);
 	if (queued)
@@ -7164,7 +7169,9 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 			p->rt.timeout = 0;
 	}
 
-	__setscheduler_prio(p, prio);
+	p->sched_class = next_class;
+	p->prio = prio;
+
 	check_class_changing(rq, p, prev_class);
 
 	if (queued)

kernel/sched/ext.c

@@ -4471,7 +4471,7 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
 		sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE, &ctx);
 
 		p->scx.slice = min_t(u64, p->scx.slice, SCX_SLICE_DFL);
-		__setscheduler_prio(p, p->prio);
+		p->sched_class = __setscheduler_class(p, p->prio);
 		check_class_changing(task_rq(p), p, old_class);
 
 		sched_enq_and_set_task(&ctx);
@@ -5186,7 +5186,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 
 		sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE, &ctx);
 
-		__setscheduler_prio(p, p->prio);
+		p->sched_class = __setscheduler_class(p, p->prio);
 		check_class_changing(task_rq(p), p, old_class);
 
 		sched_enq_and_set_task(&ctx);

kernel/sched/fair.c

@@ -13177,22 +13177,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
 static void switched_from_fair(struct rq *rq, struct task_struct *p)
 {
 	detach_task_cfs_rq(p);
-	/*
-	 * Since this is called after changing class, this is a little weird
-	 * and we cannot use DEQUEUE_DELAYED.
-	 */
-	if (p->se.sched_delayed) {
-		/* First, dequeue it from its new class' structures */
-		dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP);
-		/*
-		 * Now, clean up the fair_sched_class side of things
-		 * related to sched_delayed being true and that wasn't done
-		 * due to the generic dequeue not using DEQUEUE_DELAYED.
-		 */
-		finish_delayed_dequeue_entity(&p->se);
-		p->se.rel_deadline = 0;
-		__block_task(rq, p);
-	}
 }
 
 static void switched_to_fair(struct rq *rq, struct task_struct *p)

kernel/sched/sched.h

@@ -3797,7 +3797,7 @@ static inline int rt_effective_prio(struct task_struct *p, int prio)
 
 extern int __sched_setscheduler(struct task_struct *p, const struct sched_attr *attr, bool user, bool pi);
 extern int __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx);
-extern void __setscheduler_prio(struct task_struct *p, int prio);
+extern const struct sched_class *__setscheduler_class(struct task_struct *p, int prio);
 extern void set_load_weight(struct task_struct *p, bool update_load);
 extern void enqueue_task(struct rq *rq, struct task_struct *p, int flags);
 extern bool dequeue_task(struct rq *rq, struct task_struct *p, int flags);

kernel/sched/syscalls.c

@@ -529,7 +529,7 @@ int __sched_setscheduler(struct task_struct *p,
 {
 	int oldpolicy = -1, policy = attr->sched_policy;
 	int retval, oldprio, newprio, queued, running;
-	const struct sched_class *prev_class;
+	const struct sched_class *prev_class, *next_class;
 	struct balance_callback *head;
 	struct rq_flags rf;
 	int reset_on_fork;
@@ -706,6 +706,12 @@ change:
 			queue_flags &= ~DEQUEUE_MOVE;
 	}
 
+	prev_class = p->sched_class;
+	next_class = __setscheduler_class(p, newprio);
+
+	if (prev_class != next_class && p->se.sched_delayed)
+		dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
+
 	queued = task_on_rq_queued(p);
 	running = task_current(rq, p);
 	if (queued)
@@ -713,11 +719,10 @@ change:
 	if (running)
 		put_prev_task(rq, p);
 
-	prev_class = p->sched_class;
-
 	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
 		__setscheduler_params(p, attr);
-		__setscheduler_prio(p, newprio);
+		p->sched_class = next_class;
+		p->prio = newprio;
 	}
 	__setscheduler_uclamp(p, attr);
 	check_class_changing(rq, p, prev_class);