sched/deadline: Don't count nr_running for dl_server proxy tasks

JIRA: https://issues.redhat.com/browse/RHEL-110301

commit 52d1552
Author: Yicong Yang <yangyicong@hisilicon.com>
Date:   Fri Jun 27 11:54:20 2025 +0800

    sched/deadline: Don't count nr_running for dl_server proxy tasks

    On CPU offline the kernel stalled with below call trace:

      INFO: task kworker/0:1:11 blocked for more than 120 seconds.

    cpuhp hold the cpu hotplug lock endless and stalled vmstat_shepherd.
    This is because we count nr_running twice on cpuhp enqueuing and failed
    the wait condition of cpuhp:

      enqueue_task_fair() // pick cpuhp from idle, rq->nr_running = 0
        dl_server_start()
          [...]
          add_nr_running() // rq->nr_running = 1
        add_nr_running() // rq->nr_running = 2
      [switch to cpuhp, waiting on balance_hotplug_wait()]
      rcuwait_wait_event(rq->nr_running == 1 && ...) // failed, rq->nr_running=2
        schedule() // wait again

    It doesn't make sense to count the dl_server towards runnable tasks,
    since it runs other tasks.

    Fixes: 63ba842 ("sched/deadline: Introduce deadline servers")
    Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lore.kernel.org/r/20250627035420.37712-1-yangyicong@huawei.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/deadline.c

@@ -1851,7 +1851,9 @@ void inc_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 	u64 deadline = dl_se->deadline;
 
 	dl_rq->dl_nr_running++;
-	add_nr_running(rq_of_dl_rq(dl_rq), 1);
+
+	if (!dl_server(dl_se))
+		add_nr_running(rq_of_dl_rq(dl_rq), 1);
 
 	inc_dl_deadline(dl_rq, deadline);
 }
@@ -1861,7 +1863,9 @@ void dec_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 {
 	WARN_ON(!dl_rq->dl_nr_running);
 	dl_rq->dl_nr_running--;
-	sub_nr_running(rq_of_dl_rq(dl_rq), 1);
+
+	if (!dl_server(dl_se))
+		sub_nr_running(rq_of_dl_rq(dl_rq), 1);
 
 	dec_dl_deadline(dl_rq, dl_se->deadline);
 }