ANDROID: sched: Track wake_q length

Some partners have value-adds based on aosp/540066, which cannot be
carried in ACK in its entirety as it no longer makes sense as-is (the
select_idle_capacity() rework upstream solved the issue differently).

It seems that those partners do not actually need the wake-wide tweaks,
they only need to access the wake_q length for wake-up balance. To
support this, add minimal tracking to the wake_q infrastructure in the
core kernel, but do that by adding a pointer to the wake_q_head to
task_struct directly to not litter all sched classes with an additional
sibling_count_hint argument to the select_task_rq callbacks.

Modules needing to access the wake_q length can do so by dereferencing
p->wake_q_head in the wake-up path when it is non-NULL.

Bug: 173981591
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I9a98167face92e70aba847d9f04d0c216065478c

kernel/sched/core.c

@@ -538,6 +538,7 @@ static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
 	 */
 	*head->lastp = node;
 	head->lastp = &node->next;
+	head->count++;
 	return true;
 }
 
@@ -594,12 +595,14 @@ void wake_up_q(struct wake_q_head *head)
 		/* Task can safely be re-inserted now: */
 		node = node->next;
 		task->wake_q.next = NULL;
+		task->wake_q_head = head;
 
 		/*
 		 * wake_up_process() executes a full barrier, which pairs with
 		 * the queueing in wake_q_add() so as not to miss wakeups.
 		 */
 		wake_up_process(task);
+		task->wake_q_head = NULL;
 		put_task_struct(task);
 	}
 }