Re: [PATCH] sched/deadline: Fix stale dl_defer_running in update_dl_entity() if-branch

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 07, 2026 at 02:22:58PM +0200, Juri Lelli wrote:

> > Random brain wave...
> > 
> > Since the dl_server is LLF (deferred), it will pretty much always trip
> > the dl_entity_overflow() when interrupted, right? Does it make sense to
> > use the revised wake-up rule for it, when appropriate?
> > 
> > ---
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index d08b00429323..674de6a48551 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1027,7 +1027,7 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
> >  	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
> >  	    dl_entity_overflow(dl_se, rq_clock(rq))) {
> >  
> > -		if (unlikely(!dl_is_implicit(dl_se) &&
> > +		if (unlikely((!dl_is_implicit(dl_se) || dl_se->dl_defer) &&
> >  			     !dl_time_before(dl_se->deadline, rq_clock(rq)) &&
> >  			     !is_dl_boosted(dl_se))) {
> >  			update_dl_revised_wakeup(dl_se, rq);
> > 
> 
> So to keep boosting, by reducing runtime appropriately, until the end of
> the current dl-server period. Makes sense to me.

OK, like so then.

---
Subject: sched/deadline: Use revised wakeup rule for dl_server
From: Peter Zijlstra <peterz@infradead.org>
Date: Sat, 4 Apr 2026 12:22:44 +0200

John noted that commit 115135422562 ("sched/deadline: Fix 'stuck' dl_server")
unfixed the issue from commit a3a70caf7906 ("sched/deadline: Fix dl_server
behaviour").

The issue in commit 115135422562 was for wakeups of the server after the
deadline; in which case you *have* to start a new period. The case for
a3a70caf7906 is wakeups before the deadline.

Now, because the server is effectively running a least-laxity policy, it means
that any wakeup during the runnable phase means dl_entity_overflow() will be
true. This means we need to adjust the runtime to allow it to still run until
the existing deadline expires.

Use the revised wakeup rule for dl_defer entities.

Fixes: 115135422562 ("sched/deadline: Fix 'stuck' dl_server")
Reported-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Tested-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260404102244.GB22575@noisy.programming.kicks-ass.net
---
 kernel/sched/deadline.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1027,7 +1027,7 @@ static void update_dl_entity(struct sche
 	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
 	    dl_entity_overflow(dl_se, rq_clock(rq))) {
 
-		if (unlikely(!dl_is_implicit(dl_se) &&
+		if (unlikely((!dl_is_implicit(dl_se) || dl_se->dl_defer) &&
 			     !dl_time_before(dl_se->deadline, rq_clock(rq)) &&
 			     !is_dl_boosted(dl_se))) {
 			update_dl_revised_wakeup(dl_se, rq);