On Wed, Oct 08, 2025 at 11:39:11PM +0530, Shrikanth Hegde wrote:

quoted

*It pointed to this*

NIP [c0000000001fd798] dl_server_start+0x50/0xd8
LR [c0000000001d9534] enqueue_task_fair+0x228/0x8ec
Call Trace:
[c000006684a579c0] [0000000000000001] 0x1 (unreliable)
[c000006684a579f0] [c0000000001d9534] enqueue_task_fair+0x228/0x8ec
[c000006684a57a60] [c0000000001bb344] enqueue_task+0x5c/0x1c8
[c000006684a57aa0] [c0000000001c5fc0] ttwu_do_activate+0x98/0x2fc
[c000006684a57af0] [c0000000001c671c] try_to_wake_up+0x2e0/0xa60
[c000006684a57b80] [c00000000019fb48] kthread_park+0x7c/0xf0
[c000006684a57bb0] [c00000000015fefc] takedown_cpu+0x60/0x194
[c000006684a57c00] [c000000000161924] cpuhp_invoke_callback+0x1f4/0x9a4
[c000006684a57c90] [c0000000001621a4] __cpuhp_invoke_callback_range+0xd0/0x188
[c000006684a57d30] [c000000000165aec] _cpu_down+0x19c/0x560
[c000006684a57df0] [c0000000001637c0] __cpu_down_maps_locked+0x2c/0x3c
[c000006684a57e10] [c00000000018a100] work_for_cpu_fn+0x38/0x54
[c000006684a57e40] [c00000000019075c] process_one_work+0x1d8/0x554
[c000006684a57ef0] [c00000000019165c] worker_thread+0x308/0x46c
[c000006684a57f90] [c00000000019e474] kthread+0x16c/0x19c
[c000006684a57fe0] [c00000000000dd58] start_kernel_thread+0x14/0x18

It is takedown_cpu called from CPU0(boot CPU) and it wakes up kthread
which is CPU Bound I guess.  Since happens after rq was marked
offline, it ends up starting the deadline server again.

So i think it is sensible idea to stop the deadline server if the cpu
is going down.  Once we stop the server we will return
HRTIMER_NORESTART.

D'0h.. that stop was far too early.

How about moving that dl_server_stop() into sched_cpu_dying() like so.

This seems to survive a few hotplugs for me.

---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 198d2dd45f59..f1ebf67b48e2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -8571,10 +8571,12 @@ int sched_cpu_dying(unsigned int cpu)
  	sched_tick_stop(cpu);
  
  	rq_lock_irqsave(rq, &rf);
+	update_rq_clock(rq);
  	if (rq->nr_running != 1 || rq_has_pinned_tasks(rq)) {
  		WARN(true, "Dying CPU not properly vacated!");
  		dump_rq_tasks(rq, KERN_WARNING);
  	}
+	dl_server_stop(&rq->fair_server);
  	rq_unlock_irqrestore(rq, &rf);
  
  	calc_load_migrate(rq);

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 615411a0a881..7b7671060bf9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c

@@ -1582,6 +1582,9 @@ void dl_server_start(struct sched_dl_entity *dl_se)
  	if (!dl_server(dl_se) || dl_se->dl_server_active)
  		return;
  
+	if (WARN_ON_ONCE(!cpu_online(cpu_of(rq))))
+		return;
+
  	dl_se->dl_server_active = 1;
  	enqueue_dl_entity(dl_se, ENQUEUE_WAKEUP);
  	if (!dl_task(dl_se->rq->curr) || dl_entity_preempt(dl_se, &rq->curr->dl))