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

[Peter Zijlstra <peterz@infradead.org>]

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