Commit 7ea98dfa authored Feb 06, 2023 by Valentin Schneider Committed by Peter Zijlstra Feb 11, 2023

sched/deadline: Add more reschedule cases to prio_changed_dl()

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

  2972e305 ("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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/20230206140612.701871-1-vschneid@redhat.com

parent 829c1651

kernel/sched/deadline.c

+27 −15

Original line number	Diff line number	Diff line
		@@ -2663,7 +2663,9 @@ 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
		@@ -2674,6 +2676,7 @@ static void prio_changed_dl(struct rq rq, struct task_struct p,
		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.
		*/
		if (!dl_task(rq->curr) \|\|
		dl_time_before(p->dl.deadline, rq->curr->dl.deadline))
		resched_curr(rq);
		#endif /* CONFIG_SMP */
		}
		#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) = {