Add some checks to the sched_change pattern to validate assumptions
around changing classes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c  |   13 +++++++++++++
 kernel/sched/sched.h |    1 +
 2 files changed, 14 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10806,6 +10806,7 @@ struct sched_change_ctx *sched_change_be
 
 	*ctx = (struct sched_change_ctx){
 		.p = p,
+		.class = p->sched_class,
 		.flags = flags,
 		.queued = task_on_rq_queued(p),
 		.running = task_current_donor(rq, p),
@@ -10836,6 +10837,11 @@ void sched_change_end(struct sched_chang
 
 	lockdep_assert_rq_held(rq);
 
+	/*
+	 * Changing class without *QUEUE_CLASS is bad.
+	 */
+	WARN_ON_ONCE(p->sched_class != ctx->class && !(ctx->flags & ENQUEUE_CLASS));
+
 	if ((ctx->flags & ENQUEUE_CLASS) && p->sched_class->switching_to)
 		p->sched_class->switching_to(rq, p);
 
@@ -10847,6 +10853,13 @@ void sched_change_end(struct sched_chang
 	if (ctx->flags & ENQUEUE_CLASS) {
 		if (p->sched_class->switched_to)
 			p->sched_class->switched_to(rq, p);
+
+		/*
+		 * If this was a degradation in class someone should have set
+		 * need_resched by now.
+		 */
+		WARN_ON_ONCE(sched_class_above(ctx->class, p->sched_class) &&
+			     !test_tsk_need_resched(p));
 	} else {
 		p->sched_class->prio_changed(rq, p, ctx->prio);
 	}
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -4027,6 +4027,7 @@ extern void balance_callbacks(struct rq
 struct sched_change_ctx {
 	u64			prio;
 	struct task_struct	*p;
+	const struct sched_class *class;
 	int			flags;
 	bool			queued;
 	bool			running;