Re: [tip: sched/urgent] sched/deadline: Fix dl_server getting stuck

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 23.09.2025 09:25, Peter Zijlstra wrote:
> On Mon, Sep 22, 2025 at 11:57:02PM +0200, Marek Szyprowski wrote:
>> On 18.09.2025 08:56, tip-bot2 for Peter Zijlstra wrote:
>>> The following commit has been merged into the sched/urgent branch of tip:
>>>
>>> Commit-ID:     077e1e2e0015e5ba6538d1c5299fb299a3a92d60
>>> Gitweb:        https://git.kernel.org/tip/077e1e2e0015e5ba6538d1c5299fb299a3a92d60
>>> Author:        Peter Zijlstra <peterz@infradead.org>
>>> AuthorDate:    Tue, 16 Sep 2025 23:02:41 +02:00
>>> Committer:     Peter Zijlstra <peterz@infradead.org>
>>> CommitterDate: Thu, 18 Sep 2025 08:50:05 +02:00
>>>
>>> sched/deadline: Fix dl_server getting stuck
>>>
>>> John found it was easy to hit lockup warnings when running locktorture
>>> on a 2 CPU VM, which he bisected down to: commit cccb45d7c429
>>> ("sched/deadline: Less agressive dl_server handling").
>>>
>>> While debugging it seems there is a chance where we end up with the
>>> dl_server dequeued, with dl_se->dl_server_active. This causes
>>> dl_server_start() to return without enqueueing the dl_server, thus it
>>> fails to run when RT tasks starve the cpu.
>>>
>>> When this happens, dl_server_timer() catches the
>>> '!dl_se->server_has_tasks(dl_se)' case, which then calls
>>> replenish_dl_entity() and dl_server_stopped() and finally return
>>> HRTIMER_NO_RESTART.
>>>
>>> This ends in no new timer and also no enqueue, leaving the dl_server
>>> 'dead', allowing starvation.
>>>
>>> What should have happened is for the bandwidth timer to start the
>>> zero-laxity timer, which in turn would enqueue the dl_server and cause
>>> dl_se->server_pick_task() to be called -- which will stop the
>>> dl_server if no fair tasks are observed for a whole period.
>>>
>>> IOW, it is totally irrelevant if there are fair tasks at the moment of
>>> bandwidth refresh.
>>>
>>> This removes all dl_se->server_has_tasks() users, so remove the whole
>>> thing.
>>>
>>> Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling")
>>> Reported-by: John Stultz <jstultz@google.com>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Tested-by: John Stultz <jstultz@google.com>
>>> ---
>> This patch landed in today's linux-next as commit 077e1e2e0015
>> ("sched/deadline: Fix dl_server getting stuck"). In my tests I found
>> that it breaks CPU hotplug on some of my systems. On 64bit
>> Exynos5433-based TM2e board I've captured the following lock dep warning
>> (which unfortunately doesn't look like really related to CPU hotplug):
> Absolutely wild guess; does something like this help?
>
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 18a30ae35441..bf78c46620a5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12972,6 +12972,8 @@ static void rq_offline_fair(struct rq *rq)
>   
>   	/* Ensure that we remove rq contribution to group share: */
>   	clear_tg_offline_cfs_rqs(rq);
> +
> +	dl_server_stop(&rq->fair_server);
>   }
>   
>   #ifdef CONFIG_SCHED_CORE

Unfortunately not, same log.

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