[PATCH] sched/deadline: Make dl-server nohz full aware

[Juri Lelli]

The dl_server_timer() causes spurious IPIs on nohz_full cores, breaking
isolation guarantees. The timer executes on a housekeeping core and
eventually calls tick_nohz_dep_set_cpu(), sending IPIs to isolated cores
even when only a single task is running.

The problem is that dl-servers are not coordinated with nohz_full tick
state. Timers can fire and send IPIs to otherwise undisturbed cores.

Fix by managing servers in sched_can_stop_tick():

- When RT tasks run with CFS/SCX tasks, start the appropriate server
  and keep the tick running
- When only RT tasks remain, stop all servers and allow tick to stop
  (except for >1 RR tasks which need the tick for round-robin)
- When only CFS/SCX tasks remain, stop all servers before stopping tick

Introduce dl_servers_stop_all() to reduce duplication and abstract
server management from core.c. Unify RT handling into one block that
handles both RR and FIFO cases.

Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
Reported-by: David Haufe <dhaufe@xxxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/CAKJHwtOw_G67edzuHVtL1xC5Vyt6StcZzihtDd0yaKudW=rwVw@xxxxxxxxxxxxxx
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
---
I had to modify my first original attempt at fixing this (please take a
look at the linked report/discussion) to also take SCX into
consideration.

FYI, I temporarily pushed the script I'm using to repro and verify the
fix here

https://github.com/jlelli/sched-deadline-tests/blob/master/test-dlserver-nohz.sh
---
 kernel/sched/core.c     | 43 +++++++++++++++++++++++--------------------
 kernel/sched/deadline.c | 14 ++++++++++++++
 kernel/sched/sched.h    |  1 +
 3 files changed, 38 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b905805bbcbe4..98759255c306b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1414,30 +1414,35 @@ static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
 
 bool sched_can_stop_tick(struct rq *rq)
 {
-	int fifo_nr_running;
-
 	/* Deadline tasks, even if single, need the tick */
 	if (rq->dl.dl_nr_running)
 		return false;
 
 	/*
-	 * If there are more than one RR tasks, we need the tick to affect the
-	 * actual RR behaviour.
+	 * If there are RT tasks, we may need the tick (for >1 RR tasks),
+	 * but we must also service lower-priority CFS/SCX tasks via dl-servers.
 	 */
-	if (rq->rt.rr_nr_running) {
-		if (rq->rt.rr_nr_running == 1)
-			return true;
-		else
+	if (rq->rt.rt_nr_running) {
+		if (rq->cfs.h_nr_queued) {
+			dl_server_start(&rq->fair_server);
+			return false;
+		}
+#ifdef CONFIG_SCHED_CLASS_EXT
+		if (rq->scx.nr_running) {
+			dl_server_start(&rq->ext_server);
+			return false;
+		}
+#endif
+		/*
+		 * Only RT tasks, no CFS/SCX. Stop servers to prevent spurious
+		 * wakeups. Tick can stop for single RR or any FIFO, but must
+		 * run for multiple RR (round-robin behavior).
+		 */
+		dl_servers_stop_all(rq);
+		if (rq->rt.rr_nr_running > 1)
 			return false;
-	}
-
-	/*
-	 * If there's no RR tasks, but FIFO tasks, we can skip the tick, no
-	 * forced preemption between FIFO tasks.
-	 */
-	fifo_nr_running = rq->rt.rt_nr_running - rq->rt.rr_nr_running;
-	if (fifo_nr_running)
 		return true;
+	}
 
 	/*
 	 * If there are no DL,RR/FIFO tasks, there must only be CFS or SCX tasks
@@ -1462,6 +1467,7 @@ bool sched_can_stop_tick(struct rq *rq)
 			return false;
 	}
 
+	dl_servers_stop_all(rq);
 	return true;
 }
 #endif /* CONFIG_NO_HZ_FULL */
@@ -8810,10 +8816,7 @@ int sched_cpu_dying(unsigned int cpu)
 		WARN(true, "Dying CPU not properly vacated!");
 		dump_rq_tasks(rq, KERN_WARNING);
 	}
-	dl_server_stop(&rq->fair_server);
-#ifdef CONFIG_SCHED_CLASS_EXT
-	dl_server_stop(&rq->ext_server);
-#endif
+	dl_servers_stop_all(rq);
 	rq_unlock_irqrestore(rq, &rf);
 
 	calc_load_migrate(rq);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index edca7849b165d..c2b3d6bbe4828 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1826,6 +1826,20 @@ void dl_server_stop(struct sched_dl_entity *dl_se)
 	dl_se->dl_server_active = 0;
 }
 
+/*
+ * Stop all dl-servers on this runqueue. Called when transitioning to a state
+ * where the tick can be stopped (e.g., single RR/FIFO task, or no RT tasks).
+ * This ensures server timers are disarmed and won't cause spurious wakeups on
+ * nohz_full isolated cores.
+ */
+void dl_servers_stop_all(struct rq *rq)
+{
+	dl_server_stop(&rq->fair_server);
+#ifdef CONFIG_SCHED_CLASS_EXT
+	dl_server_stop(&rq->ext_server);
+#endif
+}
+
 void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_pick_f pick_task)
 {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9f63b15d309d1..26cf1d14efde5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -412,6 +412,7 @@ extern void dl_server_update_idle(struct sched_dl_entity *dl_se, s64 delta_exec)
 extern void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec);
 extern void dl_server_start(struct sched_dl_entity *dl_se);
 extern void dl_server_stop(struct sched_dl_entity *dl_se);
+extern void dl_servers_stop_all(struct rq *rq);
 extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_pick_f pick_task);
 extern void sched_init_dl_servers(void);

---
base-commit: 4ac4d6549a6563878d7c19c154e017f6cb7114d3
change-id: 20260512-upstream-fix-dlserver-nohzfull-b4-b745e2a967ed

Best regards,
--  
Juri Lelli <juri.lelli@xxxxxxxxxx>