On 10/8/25 4:43 PM, Peter Zijlstra wrote:

On Wed, Oct 08, 2025 at 03:47:16PM +0530, Shrikanth Hegde wrote:

quoted

On 10/8/25 3:20 PM, Peter Zijlstra wrote:

quoted

On Wed, Oct 08, 2025 at 07:41:10AM +0530, Venkat Rao Bagalkote wrote:

quoted

Greetings!!!


IBM CI has reported a kernel warnings while running CPU hot plug operation
on IBM Power9 system.


Command to reproduce the issue:

drmgr -c cpu -r -q 1

I do not know what drmgr is. I am not familiar with PowerPC tools.
AFAICT x86 never modifies cpu_present_mask after boot.

It is a tool which allows dynamic addition of cpu/memory. It does indeed changes the present cpus.
Even i am not profound with it :)

quoted

maybe during drmgr, the dl server gets started again? Maybe that's why above patch it didn't work.
Will see and understand this bit more.

dl_server is per cpu and is started on enqueue of a fair task when:

   - the runqueue was empty; and
   - the dl_server wasn't already active

Once the dl_server is active it has this timer (you already found this),
this timer is set for the 0-laxity moment (the last possible moment in
time where it can still run its budget and not be late), during this
time any fair runtime is accounted against it budget (subtracted from).

Once the timer fires and it still has budget left; it will enqueue the
deadline entity. However the more common case is that its budget will be
depleted, in which case the timer is reset to its period end for
replenish (where it gets new runtime budget), after which its back to
the 0-laxity.

If the deadline entity gets scheduled, it will try and pick a fair task
and run that. In the case where there is no fair task, it will
deactivate itself.

ok cool.

The patch I sent earlier would force stop the deadline timer on CPU
offline.

quoted

Also, i tried this below diff which fixes it. Just ignore the hrtimer if the cpu is offline.
Does this makes sense?
---

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 615411a0a881..a342cf5e4624 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c

@@ -1160,6 +1160,9 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_
  	scoped_guard (rq_lock, rq) {
  		struct rq_flags *rf = &scope.rf;
+		if (!cpu_online(rq->cpu))
+			return HRTIMER_NORESTART;
+
  		if (!dl_se->dl_throttled || !dl_se->dl_runtime)
  			return HRTIMER_NORESTART;

This could leave the dl_server in inconsistent state. It would have to
call dl_server_stop() or something along those lines.

Also, this really should not happen; per my previous patch we should be
stopping the timer when we go offline.

Since you can readily reproduce this; perhaps you could stick something
like this in dl_server_start():

	WARN_ON_ONCE(!cpu_online(rq->cpu))

See if anybody is (re)starting the thing ?

So i did use this diff to get who is enabling it again, after it was stopped in offline.

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 198d2dd45f59..83e77bbbb6b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -8328,6 +8328,8 @@ static inline void sched_set_rq_offline(struct rq *rq, int cpu)
                 BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
                 set_rq_offline(rq);
         }
+       dl_server_stop(&rq->fair_server);
+
         rq_unlock_irqrestore(rq, &rf);
  }
  

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 615411a0a881..5847540bdc18 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c

@@ -1582,6 +1582,8 @@ void dl_server_start(struct sched_dl_entity *dl_se)
         if (!dl_server(dl_se) || dl_se->dl_server_active)
                 return;
  
+       WARN_ON(!rq->online);
+
         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))

*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.

This does fix the warning. Does this look any good?

---

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 615411a0a881..831797b9ec0f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c

@@ -1160,11 +1160,14 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_
         scoped_guard (rq_lock, rq) {
                 struct rq_flags *rf = &scope.rf;
  
+               update_rq_clock(rq);
+               if (!cpu_online(rq->cpu))
+                       dl_server_stop(dl_se);
+
                 if (!dl_se->dl_throttled || !dl_se->dl_runtime)
                         return HRTIMER_NORESTART;
  
                 sched_clock_tick();
-               update_rq_clock(rq);
  
                 if (!dl_se->dl_runtime)
                         return HRTIMER_NORESTART;

----

Also below check is duplicate. We do the same check above.

if (!dl_se->dl_runtime)
                         return HRTIMER_NORESTART;