md: add check for sleepers in md_wakeup_thread()

Check for sleeping thread before attempting its wake_up in
md_wakeup_thread() to avoid unnecessary spinlock contention.

With a 6.1 kernel, fio random read/write tests on many (>= 100)
virtual volumes, of 100 GiB each, on 3 md-raid5s on 8 SSDs each
(building a raid50), show by 3 to 4 % improved IOPS performance.

Signed-off-by: Florian-Ewald Mueller <florian-ewald.mueller@ionos.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20240327114022.74634-1-jinpu.wang@ionos.com
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -8087,7 +8087,8 @@ void md_wakeup_thread(struct md_thread __rcu *thread)
 	if (t) {
 		pr_debug("md: waking up MD thread %s.\n", t->tsk->comm);
 		set_bit(THREAD_WAKEUP, &t->flags);
-		wake_up(&t->wqueue);
+		if (wq_has_sleeper(&t->wqueue))
+			wake_up(&t->wqueue);
 	}
 	rcu_read_unlock();
 }