Re: [RFC][PATCH 5/5] sched: Add ttwu_queue support for delayed tasks

[Peter Zijlstra]

On Fri, Jun 13, 2025 at 11:51:19AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 13, 2025 at 09:34:22AM +0200, Dietmar Eggemann wrote:
> > On 20/05/2025 11:45, Peter Zijlstra wrote:
> > 
> > [...]
> > 
> > > @@ -3830,12 +3859,41 @@ void sched_ttwu_pending(void *arg)
> > >  	update_rq_clock(rq);
> > >  
> > >  	llist_for_each_entry_safe(p, t, llist, wake_entry.llist) {
> > > +		struct rq *p_rq = task_rq(p);
> > > +		int ret;
> > > +
> > > +		/*
> > > +		 * This is the ttwu_runnable() case. Notably it is possible for
> > > +		 * on-rq entities to get migrated -- even sched_delayed ones.
> > > +		 */
> > > +		if (unlikely(p_rq != rq)) {
> > > +			rq_unlock(rq, &rf);
> > > +			p_rq = __task_rq_lock(p, &rf);
> > 
> > I always get this fairly early with TTWU_QUEUE_DELAYED enabled, related
> > to p->pi_lock not held in wakeup from interrupt.
> > 
> > [   36.175285] WARNING: CPU: 0 PID: 162 at kernel/sched/core.c:679 __task_rq_lock+0xf8/0x128
> 
> Thanks, let me go have a look.

I'm thinking this should cure things.

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -677,7 +677,12 @@ struct rq *__task_rq_lock(struct task_st
 {
 	struct rq *rq;
 
-	lockdep_assert_held(&p->pi_lock);
+	/*
+	 * TASK_WAKING is used to serialize the remote end of wakeup, rather
+	 * than p->pi_lock.
+	 */
+	lockdep_assert(p->__state == TASK_WAKING ||
+		       lockdep_is_held(&p->pi_lock) != LOCK_STATE_NOT_HELD);
 
 	for (;;) {
 		rq = task_rq(p);