sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks

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

commit b7ca574
Author: John Stultz <[email protected]>
Date:   Tue Apr 29 08:07:26 2025 -0700

    sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks

    It was reported that in 6.12, smpboot_create_threads() was
    taking much longer then in 6.6.

    I narrowed down the call path to:
     smpboot_create_threads()
     -> kthread_create_on_cpu()
        -> kthread_bind()
           -> __kthread_bind_mask()
              ->wait_task_inactive()

    Where in wait_task_inactive() we were regularly hitting the
    queued case, which sets a 1 tick timeout, which when called
    multiple times in a row, accumulates quickly into a long
    delay.

    I noticed disabling the DELAY_DEQUEUE sched feature recovered
    the performance, and it seems the newly create tasks are usually
    sched_delayed and left on the runqueue.

    So in wait_task_inactive() when we see the task
    p->se.sched_delayed, manually dequeue the sched_delayed task
    with DEQUEUE_DELAYED, so we don't have to constantly wait a
    tick.

    Fixes: 152e11f ("sched/fair: Implement delayed dequeue")
    Reported-by: [email protected]
    Signed-off-by: John Stultz <[email protected]>
    Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
    Tested-by: K Prateek Nayak <[email protected]>
    Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Phil Auld <[email protected]>

Author: auldp

kernel/sched/core.c

@@ -2256,6 +2256,12 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 		 * just go back and repeat.
 		 */
 		rq = task_rq_lock(p, &rf);
+		/*
+		 * If task is sched_delayed, force dequeue it, to avoid always
+		 * hitting the tick timeout in the queued case
+		 */
+		if (p->se.sched_delayed)
+			dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
 		trace_sched_wait_task(p);
 		running = task_on_cpu(rq, p);
 		queued = task_on_rq_queued(p);