[RFC PATCH v2] sched/deadline: Add more reschedule cases to prio_changed_dl()

[Valentin Schneider]

I've been tracking down an issue on a ~5.17ish kernel where:

  CPUx                           CPUy

  <DL task p0 owns an rtmutex M>
  <p0 depletes its runtime, gets throttled>
  <rq switches to the idle task>
				 <DL task p1 blocks on M, boost/replenish p0>
				 <No call to resched_curr() happens here>

  [idle task keeps running here until *something*
   accidentally sets TIF_NEED_RESCHED]

On that kernel, it is quite easy to trigger using rt-tests's deadline_test
[1] with the test running on isolated CPUs (this reduces the chance of
something unrelated setting TIF_NEED_RESCHED on the idle tasks, making the
issue even more obvious as the hung task detector chimes in).

I haven't been able to reproduce this using a mainline kernel, even if I
revert

  2972e3050e35 ("tracing: Make trace_marker{,_raw} stream-like")

which gets rid of the lock involved in the above test, *but* I cannot
convince myself the issue isn't there from looking at the code.

Make prio_changed_dl() issue a reschedule if the current task isn't a
deadline one. While at it, ensure a reschedule is emitted when a
queued-but-not-current task gets boosted with an earlier deadline that
current's.

[1]: https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
RFCv1 -> RFCv2
++++++++++++++
o Fixed UP build issue (Juri)
---
 kernel/sched/deadline.c | 42 ++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0d97d54276cc8..71b24371a6f77 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2663,17 +2663,20 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
 static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 			    int oldprio)
 {
-	if (task_on_rq_queued(p) || task_current(rq, p)) {
+	if (!task_on_rq_queued(p))
+		return;
+
 #ifdef CONFIG_SMP
-		/*
-		 * This might be too much, but unfortunately
-		 * we don't have the old deadline value, and
-		 * we can't argue if the task is increasing
-		 * or lowering its prio, so...
-		 */
-		if (!rq->dl.overloaded)
-			deadline_queue_pull_task(rq);
+	/*
+	 * This might be too much, but unfortunately
+	 * we don't have the old deadline value, and
+	 * we can't argue if the task is increasing
+	 * or lowering its prio, so...
+	 */
+	if (!rq->dl.overloaded)
+		deadline_queue_pull_task(rq);
 
+	if (task_current(rq, p)) {
 		/*
 		 * If we now have a earlier deadline task than p,
 		 * then reschedule, provided p is still on this
@@ -2681,15 +2684,24 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 		 */
 		if (dl_time_before(rq->dl.earliest_dl.curr, p->dl.deadline))
 			resched_curr(rq);
-#else
+	} else {
 		/*
-		 * Again, we don't know if p has a earlier
-		 * or later deadline, so let's blindly set a
-		 * (maybe not needed) rescheduling point.
+		 * Current may not be deadline in case p was throttled but we
+		 * have just replenished it (e.g. rt_mutex_setprio()).
+		 *
+		 * Otherwise, if p was given an earlier deadline, reschedule.
 		 */
-		resched_curr(rq);
-#endif /* CONFIG_SMP */
+		if (!dl_task(rq->curr) ||
+		    dl_time_before(p->dl.deadline, rq->curr->dl.deadline))
+			resched_curr(rq);
 	}
+#else
+	/*
+	 * We don't know if p has a earlier or later deadline, so let's blindly
+	 * set a (maybe not needed) rescheduling point.
+	 */
+	resched_curr(rq);
+#endif
 }
 
 DEFINE_SCHED_CLASS(dl) = {
-- 
2.31.1