Commit 11513542 authored Jan 30, 2026 by Peter Zijlstra

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the dl_server's job is no longer valid at the time of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: a110a81c ("sched/deadline: Deferrable dl server")
Reported-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Tested-by: Andrea Righi <arighi@nvidia.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

parent 63804fed

kernel/sched/deadline.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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;