linux-kernel - [PATCH] sched/deadline: stop dl_server before CPU goes offline

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Message-ID: <20251009184727.673081-1-sshegde@linux.ibm.com>
Date: Fri, 10 Oct 2025 00:17:27 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: peterz@...radead.org, juri.lelli@...hat.com
Cc: sshegde@...ux.ibm.com, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        m.szyprowski@...sung.com, venkat88@...ux.ibm.com, jstultz@...gle.com
Subject: [PATCH] sched/deadline: stop dl_server before CPU goes offline

From: Peter Zijlstra (Intel) <peterz@...radead.org>

IBM CI tool reported kernel warning[1] when running a CPU removal
operation through drmgr[2]. i.e "drmgr -c cpu -r -q 1"

WARNING: CPU: 0 PID: 0 at kernel/sched/cpudeadline.c:219 cpudl_set+0x58/0x170
NIP [c0000000002b6ed8] cpudl_set+0x58/0x170
LR [c0000000002b7cb8] dl_server_timer+0x168/0x2a0
Call Trace:
[c000000002c2f8c0] init_stack+0x78c0/0x8000 (unreliable)
[c0000000002b7cb8] dl_server_timer+0x168/0x2a0
[c00000000034df84] __hrtimer_run_queues+0x1a4/0x390
[c00000000034f624] hrtimer_interrupt+0x124/0x300
[c00000000002a230] timer_interrupt+0x140/0x320

Git bisects to: commit 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")

This happens since: 
- dl_server hrtimer gets enqueued close to cpu offline, when 
  kthread_park enqueues a fair task.
- CPU goes offline and drmgr removes it from cpu_present_mask.
- hrtimer fires and warning is hit.

Fix it by stopping the dl_server before CPU is marked dead.

[1]: https://lore.kernel.org/all/8218e149-7718-4432-9312-f97297c352b9@linux.ibm.com/
[2]: https://github.com/ibm-power-utilities/powerpc-utils/tree/next/src/drmgr

[sshegde: wrote the changelog and tested it]
Fixes: 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
Reported-by: Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Closes: https://lore.kernel.org/all/8218e149-7718-4432-9312-f97297c352b9@linux.ibm.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>
Tested-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 kernel/sched/core.c     | 2 ++
 kernel/sched/deadline.c | 3 +++
 2 files changed, 5 insertions(+)

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))
-- 
2.47.3