[RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline

[Gabriele Monaco]

Recent changes to the deadline server leave it running when the system
is idle. If the system is idle for longer than the dl_server period and
the first scheduling occurs after a fair task wakes up, the algorithm
picks the server as the earliest deadline (in the past) and that boosts
the fair task that just woke up while:
 * the deadline is in the past
 * the server consumed all its runtime (in background)
 * there is no starvation (idle for about a period)

Prevent the server from boosting a task when the deadline is in the
past. Instead, replenish a new period and start the server as deferred.

Fixes: 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
To: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Clark Williams <williams@xxxxxxxxxx>
Signed-off-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
---

This behaviour was observed using the RV monitors in [1] and the patch
was validated on an adapted version of the models. The models are not
exhaustively validating the dl_server behaviour.

[1] - https://lore.kernel.org/lkml/20250919140954.104920-21-gmonaco@xxxxxxxxxx

 kernel/sched/deadline.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 72c1f72463c7..b3e3d506a18d 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2371,6 +2371,17 @@ static struct task_struct *__pick_task_dl(struct rq *rq)
 			dl_server_stop(dl_se);
 			goto again;
 		}
+		/*
+		 * If the CPU was idle for long enough time and wakes up
+		 * because of a fair task, the dl_server may run after its
+		 * period elapsed. Replenish a new period as deferred, since we
+		 * are clearly not handling starvation here.
+		 */
+		if (dl_time_before(dl_se->deadline, rq_clock(rq))) {
+			dl_se->dl_defer_running = 0;
+			replenish_dl_new_period(dl_se, rq);
+			goto again;
+		}
 		rq->dl_server = dl_se;
 	} else {
 		p = dl_task_of(dl_se);

base-commit: e5f0a698b34ed76002dc5cff3804a61c80233a7a
-- 
2.51.0