[PATCH v2] sched: allow resubmits to queue_balance_callback()

[Barret Rhoden <brho@google.com>]

Prior to this commit, if you submitted the same callback_head twice, it
would be enqueued twice, but only if it was the last callback on the
list.  The first time it was submitted, rq->balance_callback was NULL,
so head->next is NULL.  That defeated the check in
queue_balance_callback().

This commit changes the callback list such that whenever an item is on
the list, its head->next is not NULL.  The last element (first inserted)
will point to itself.  This allows us to detect and ignore any attempt
to reenqueue a callback_head.

Signed-off-by: Barret Rhoden <brho@google.com>
---

sorry about the old version.  i updated to linus's branch, plus a minor
fix from v1.

this should work with the balance_push_callback stuff you added a few
months ago.  in this commit, head->next can equal null *or* head and it
will be treated as null.  (balance_push_callback's next == NULL).

thanks,
barret


 kernel/sched/core.c  | 3 ++-
 kernel/sched/sched.h | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 98191218d891..c5a1a225d0b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3986,7 +3986,8 @@ static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
 
 	while (head) {
 		func = (void (*)(struct rq *))head->func;
-		next = head->next;
+		/* The last element pointed to itself */
+		next = head->next == head ? NULL : head->next;
 		head->next = NULL;
 		head = next;
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 10a1522b1e30..66e1a9e5a1af 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1418,11 +1418,15 @@ queue_balance_callback(struct rq *rq,
 {
 	lockdep_assert_held(&rq->lock);
 
+	/*
+	 * The last element on the list points to itself, so we can always
+	 * detect if head is already enqueued.
+	 */
 	if (unlikely(head->next || rq->balance_callback == &balance_push_callback))
 		return;
 
 	head->func = (void (*)(struct callback_head *))func;
-	head->next = rq->balance_callback;
+	head->next = rq->balance_callback ?: head;
 	rq->balance_callback = head;
 }
 
-- 
2.31.0.rc2.261.g7f71774620-goog