linux-kernel - [tip: sched/urgent] sched/deadline: Fix 'stuck' dl

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Message-ID: <176981112143.2495410.4348604834756100373.tip-bot2@tip-bot2>
Date: Fri, 30 Jan 2026 22:12:01 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Andrea@...-bot2.tec.linutronix.de,
	Righi@...-bot2.tec.linutronix.de, arighi@...dia.com,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/deadline: Fix 'stuck' dl_server

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     115135422562e2f791e98a6f55ec57b2da3b3a95
Gitweb:        https://git.kernel.org/tip/115135422562e2f791e98a6f55ec57b2da3b3a95
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Fri, 30 Jan 2026 13:41:00 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 30 Jan 2026 23:06:06 +01:00

sched/deadline: Fix 'stuck' dl_server

Andrea reported the dl_server getting stuck for him. He tracked it
down to a state where dl_server_start() saw dl_defer_running==1, but
the dl_server's job is no longer valid at the time of
dl_server_start().

In the state diagram this corresponds to [4] D->A (or dl_server_stop()
due to no more runnable tasks) followed by [1], which in case of a
lapsed deadline must then be A->B.

Now our A has dl_defer_running==1, while B demands
dl_defer_running==0, therefore it must get cleared when the CBS wakeup
rules demand a replenish.

Fixes: a110a81c52a9 ("sched/deadline: Deferrable dl server")
Reported-by: Andrea Righi arighi@...dia.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Juri Lelli <juri.lelli@...hat.com>
Tested-by: Andrea Righi arighi@...dia.com
Link: https://lkml.kernel.org/r/20260123161645.2181752-1-arighi@nvidia.com
Link: https://patch.msgid.link/20260130124100.GC1079264@noisy.programming.kicks-ass.net
---
 kernel/sched/deadline.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index c509f2e..7bcde71 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1034,6 +1034,12 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
 			return;
 		}
 
+		/*
+		 * When [4] D->A is followed by [1] A->B, dl_defer_running
+		 * needs to be cleared, otherwise it will fail to properly
+		 * start the zero-laxity timer.
+		 */
+		dl_se->dl_defer_running = 0;
 		replenish_dl_new_period(dl_se, rq);
 	} else if (dl_server(dl_se) && dl_se->dl_defer) {
 		/*
@@ -1655,6 +1661,12 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
+ *       if (dl_time_before() || dl_entity_overflow())
+ *         dl_defer_running = 0;
+ *         replenish_dl_new_period();
+ *           // fwd period
+ *           dl_throttled = 1;
+ *           dl_defer_armed = 1;
  *       if (!dl_defer_running)
  *         dl_defer_armed = 1;
  *         dl_throttled = 1;