linux-kernel - Re: [PATCH] sched/deadline: stop dl

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Message-ID: <58f317d8-3137-4847-9a27-cbdf0f54eff2@samsung.com>
Date: Thu, 9 Oct 2025 22:28:32 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, peterz@...radead.org,
	juri.lelli@...hat.com
Cc: mingo@...hat.com, vincent.guittot@...aro.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	venkat88@...ux.ibm.com, jstultz@...gle.com
Subject: Re: [PATCH] sched/deadline: stop dl_server before CPU goes offline

On 09.10.2025 20:47, Shrikanth Hegde wrote:
> 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
Closes: 
https://lore.kernel.org/all/e56310b5-f7a9-4fad-b79a-dcbcdd3d3883@samsung.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))

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland