Subject: Re: [BUG almost bisected] Splat in dequeue_rt_stack() and build error

From: Peter Zijlstra <[email-masked]>
Date: Thu, 3 Oct 2024 20:50:37 +0200

On Thu, Oct 03, 2024 at 09:04:30AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 03, 2024 at 04:22:40PM +0200, Peter Zijlstra wrote:
> > On Thu, Oct 03, 2024 at 05:45:47AM -0700, Paul E. McKenney wrote:
> > 
> > > I ran 100*TREE03 for 18 hours each, and got 23 instances of *something*
> > > happening (and I need to suppress stalls on the repeat).  One of the
> > > earlier bugs happened early, but sadly not this one.
> > 
> > Damn, I don't have the amount of CPU hours available you mention in your
> > later email. I'll just go up the rounds to 20 minutes and see if
> > something wants to go bang before I have to shut down the noise
> > pollution for the day...
> 
> Indeed, this was one reason I was soliciting debug patches.  ;-)

Sooo... I was contemplating if something like the below might perhaps
help some. It's a bit of a mess (I'll try and clean up if/when it
actually proves to work), but it compiles and survives a hand full of 1m
runs.

I'll try and give it more runs tomorrow when I can power up the big
machines again -- unless you've already told me it's crap by then :-)

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 43e453ab7e20..1fe850788195 100644
--- a/kernel/sched/core.c
+++ b/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;
+		return &dl_sched_class;
+
+	if (rt_prio(prio))
+		return &rt_sched_class;
+
 #ifdef CONFIG_SCHED_CLASS_EXT
-	else if (task_should_scx(p))
-		p->sched_class = &ext_sched_class;
+	if (task_should_scx(p))
+		return &ext_sched_class;
 #endif
-	else
-		p->sched_class = &fair_sched_class;
 
-	p->prio = prio;
+	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)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ab497fafa7be..c157d4860a3b 100644
--- a/kernel/sched/fair.c
+++ b/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)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b1c3588a8f00..fba524c81c63 100644
--- a/kernel/sched/sched.h
+++ b/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);
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index aa70beee9895..0470bcc3d204 100644
--- a/kernel/sched/syscalls.c
+++ b/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 @@ int __sched_setscheduler(struct task_struct *p,
 			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 @@ int __sched_setscheduler(struct task_struct *p,
 	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);