[patch] Re: [PowerPC][Linux-next][6.11.0-rc4-next-20240820] OOPs while running LTP FS Stress

[Mike Galbraith <efault@gmx.de>]

On Mon, 2024-09-16 at 12:00 +0530, Venkat Rao Bagalkote wrote:
> Greetings!!!

Greetings,

> I am seeing below kernel crash from 6.11.0-rc4-next-20240820.
>
>
> Tried to do git bisect, but it didnt point to right patch. Attached is
> the bisect log.
>
> Any help in fixing this is much appriciated.

I met this, as well as other ways the wheels can fall off that turned
out to have the same root.  I gave Peter a heads up with diag offline,
but having now convinced myself that all is well, I'll go ahead and
post a patchlet.

At the very least it's worth putting out for wider testing.. and should
anyone have something prettier in mind, yeah, do that instead.

sched: Fix sched_delayed vs cfs_bandwidth

Meeting an unfinished DELAY_DEQUEUE treated entity in unthrottle_cfs_rq()
leads to a couple terminal scenarios.  Finish it first, so ENQUEUE_WAKEUP
can proceed as it would have sans DELAY_DEQUEUE treatment.

Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Signed-off-by: Mike Galbraith <efault@gmx.de>
---
 kernel/sched/fair.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6049,10 +6049,14 @@ void unthrottle_cfs_rq(struct cfs_rq *cf
 	for_each_sched_entity(se) {
 		struct cfs_rq *qcfs_rq = cfs_rq_of(se);

-		if (se->on_rq) {
-			SCHED_WARN_ON(se->sched_delayed);
-			break;
+		/* Handle any unfinished DELAY_DEQUEUE business first. */
+		if (unlikely(se->on_rq && se->sched_delayed)) {
+			int flags = DEQUEUE_SLEEP | DEQUEUE_SPECIAL;
+
+			dequeue_entity(qcfs_rq, se, flags | DEQUEUE_DELAYED);
 		}
+		if (se->on_rq)
+			break;
 		enqueue_entity(qcfs_rq, se, ENQUEUE_WAKEUP);

 		if (cfs_rq_is_idle(group_cfs_rq(se)))