[RFC][PATCH v2] sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks

[John Stultz <jstultz@google.com>]

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.

This seems to work, but I've only lightly tested it, so I'd love
close review and feedback in case I've missed something in
wait_task_inactive(), or if there is a simpler alternative
approach.

NOTE: Peter did highlight[1] his general distaste for the
kthread_bind() through wait_task_inactive() functions, which
suggests a deeper rework might be better, but I'm not familiar
enough with all its users to have a sense of how that might be
done, and this fix seems to address the problem and be more
easily backported to 6.12-stable, so I wanted to submit it
again, as a potentially more short-term solution.

[1]: https://lore.kernel.org/lkml/20250422085628.GA14170@noisy.programming.kicks-ass.net/

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: kernel-team@android.com
Reported-by: peter-yc.chang@mediatek.com
Signed-off-by: John Stultz <jstultz@google.com>
---
v2:
* Rework & simplify the check as suggested by K Prateek Nayak
* Added Reported-by tag for proper attribution
---
 kernel/sched/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c81cf642dba05..b986cd2fb19b7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2283,6 +2283,12 @@ 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);
-- 
2.49.0.850.g28803427d3-goog