lib/timerqueue: Rely on rbtree semantics for next timer

Simplify the timerqueue code by using cached rbtrees and rely on the tree
leftmost node semantics to get the timer with earliest expiration time.
This is a drop in conversion, and therefore semantics remain untouched.

The runtime overhead of cached rbtrees is be pretty much the same as the
current head->next method, noting that when removing the leftmost node,
a common operation for the timerqueue, the rb_next(leftmost) is O(1) as
well, so the next timer will either be the right node or its parent.
Therefore no extra pointer chasing. Finally, the size of the struct
timerqueue_head remains the same.

Passes several hours of rcutorture.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190724152323.bojciei3muvfxalm@linux-r8p5

include/linux/timerqueue.h

@@ -12,8 +12,7 @@ struct timerqueue_node {
 };
 
 struct timerqueue_head {
-	struct rb_root head;
-	struct timerqueue_node *next;
+	struct rb_root_cached rb_root;
 };
 
 
@@ -29,13 +28,14 @@ extern struct timerqueue_node *timerqueue_iterate_next(
  *
  * @head: head of timerqueue
  *
- * Returns a pointer to the timer node that has the
- * earliest expiration time.
+ * Returns a pointer to the timer node that has the earliest expiration time.
  */
 static inline
 struct timerqueue_node *timerqueue_getnext(struct timerqueue_head *head)
 {
-	return head->next;
+	struct rb_node *leftmost = rb_first_cached(&head->rb_root);
+
+	return rb_entry(leftmost, struct timerqueue_node, node);
 }
 
 static inline void timerqueue_init(struct timerqueue_node *node)
@@ -45,7 +45,6 @@ static inline void timerqueue_init(struct timerqueue_node *node)
 
 static inline void timerqueue_init_head(struct timerqueue_head *head)
 {
-	head->head = RB_ROOT;
-	head->next = NULL;
+	head->rb_root = RB_ROOT_CACHED;
 }
 #endif /* _LINUX_TIMERQUEUE_H */

lib/timerqueue.c

@@ -26,9 +26,10 @@
  */
 bool timerqueue_add(struct timerqueue_head *head, struct timerqueue_node *node)
 {
-	struct rb_node **p = &head->head.rb_node;
+	struct rb_node **p = &head->rb_root.rb_root.rb_node;
 	struct rb_node *parent = NULL;
-	struct timerqueue_node  *ptr;
+	struct timerqueue_node *ptr;
+	bool leftmost = true;
 
 	/* Make sure we don't add nodes that are already added */
 	WARN_ON_ONCE(!RB_EMPTY_NODE(&node->node));
@@ -36,19 +37,17 @@ bool timerqueue_add(struct timerqueue_head *head, struct timerqueue_node *node)
 	while (*p) {
 		parent = *p;
 		ptr = rb_entry(parent, struct timerqueue_node, node);
-		if (node->expires < ptr->expires)
+		if (node->expires < ptr->expires) {
 			p = &(*p)->rb_left;
-		else
+		} else {
 			p = &(*p)->rb_right;
+			leftmost = false;
+		}
 	}
 	rb_link_node(&node->node, parent, p);
-	rb_insert_color(&node->node, &head->head);
+	rb_insert_color_cached(&node->node, &head->rb_root, leftmost);
 
-	if (!head->next || node->expires < head->next->expires) {
-		head->next = node;
-		return true;
-	}
-	return false;
+	return leftmost;
 }
 EXPORT_SYMBOL_GPL(timerqueue_add);
 
@@ -65,15 +64,10 @@ bool timerqueue_del(struct timerqueue_head *head, struct timerqueue_node *node)
 {
 	WARN_ON_ONCE(RB_EMPTY_NODE(&node->node));
 
-	/* update next pointer */
-	if (head->next == node) {
-		struct rb_node *rbn = rb_next(&node->node);
-
-		head->next = rb_entry_safe(rbn, struct timerqueue_node, node);
-	}
-	rb_erase(&node->node, &head->head);
+	rb_erase_cached(&node->node, &head->rb_root);
 	RB_CLEAR_NODE(&node->node);
-	return head->next != NULL;
+
+	return !RB_EMPTY_ROOT(&head->rb_root.rb_root);
 }
 EXPORT_SYMBOL_GPL(timerqueue_del);