[PATCH] cfq-iosched: speed up rbtree handling

For cases where the rbtree is mainly used for sorting and min retrieval,
a nice speedup of the rbtree code is to maintain a cache of the leftmost
node in the tree.

Also spotted in the CFS CPU scheduler code.

Improved by Alan D. Brunelle <Alan.Brunelle@hp.com> by updating the
leftmost hint in cfq_rb_first() if it isn't set, instead of only
updating it on insert.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -69,6 +69,18 @@ static struct completion *ioc_gone;
 
 #define sample_valid(samples)	((samples) > 80)
 
+/*
+ * Most of our rbtree usage is for sorting with min extraction, so
+ * if we cache the leftmost node we don't have to walk down the tree
+ * to find it. Idea borrowed from Ingo Molnars CFS scheduler. We should
+ * move this into the elevator for the rq sorting as well.
+ */
+struct cfq_rb_root {
+	struct rb_root rb;
+	struct rb_node *left;
+};
+#define CFQ_RB_ROOT	(struct cfq_rb_root) { RB_ROOT, NULL, }
+
 /*
  * Per block device queue structure
  */
@@ -78,7 +90,7 @@ struct cfq_data {
 	/*
 	 * rr list of queues with requests and the count of them
 	 */
-	struct rb_root service_tree;
+	struct cfq_rb_root service_tree;
 	struct list_head cur_rr;
 	struct list_head idle_rr;
 	unsigned int busy_queues;
@@ -378,6 +390,23 @@ cfq_choose_req(struct cfq_data *cfqd, struct request *rq1, struct request *rq2)
 	}
 }
 
+static struct rb_node *cfq_rb_first(struct cfq_rb_root *root)
+{
+	if (!root->left)
+		root->left = rb_first(&root->rb);
+
+	return root->left;
+}
+
+static void cfq_rb_erase(struct rb_node *n, struct cfq_rb_root *root)
+{
+	if (root->left == n)
+		root->left = NULL;
+
+	rb_erase(n, &root->rb);
+	RB_CLEAR_NODE(n);
+}
+
 /*
  * would be nice to take fifo expire time into account as well
  */
@@ -417,10 +446,10 @@ static unsigned long cfq_slice_offset(struct cfq_data *cfqd,
 static void cfq_service_tree_add(struct cfq_data *cfqd,
 				    struct cfq_queue *cfqq)
 {
-	struct rb_node **p = &cfqd->service_tree.rb_node;
+	struct rb_node **p = &cfqd->service_tree.rb.rb_node;
 	struct rb_node *parent = NULL;
-	struct cfq_queue *__cfqq;
 	unsigned long rb_key;
+	int left = 1;
 
 	rb_key = cfq_slice_offset(cfqd, cfqq) + jiffies;
 	rb_key += cfqq->slice_resid;
@@ -433,22 +462,29 @@ static void cfq_service_tree_add(struct cfq_data *cfqd,
 		if (rb_key == cfqq->rb_key)
 			return;
 
-		rb_erase(&cfqq->rb_node, &cfqd->service_tree);
+		cfq_rb_erase(&cfqq->rb_node, &cfqd->service_tree);
 	}
 
 	while (*p) {
+		struct cfq_queue *__cfqq;
+
 		parent = *p;
 		__cfqq = rb_entry(parent, struct cfq_queue, rb_node);
 
 		if (rb_key < __cfqq->rb_key)
 			p = &(*p)->rb_left;
-		else
+		else {
 			p = &(*p)->rb_right;
+			left = 0;
 		}
+	}
+
+	if (left)
+		cfqd->service_tree.left = &cfqq->rb_node;
 
 	cfqq->rb_key = rb_key;
 	rb_link_node(&cfqq->rb_node, parent, p);
-	rb_insert_color(&cfqq->rb_node, &cfqd->service_tree);
+	rb_insert_color(&cfqq->rb_node, &cfqd->service_tree.rb);
 }
 
 static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
@@ -509,10 +545,8 @@ cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	cfq_clear_cfqq_on_rr(cfqq);
 	list_del_init(&cfqq->cfq_list);
 
-	if (!RB_EMPTY_NODE(&cfqq->rb_node)) {
-		rb_erase(&cfqq->rb_node, &cfqd->service_tree);
-		RB_CLEAR_NODE(&cfqq->rb_node);
-	}
+	if (!RB_EMPTY_NODE(&cfqq->rb_node))
+		cfq_rb_erase(&cfqq->rb_node, &cfqd->service_tree);
 
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
@@ -764,8 +798,8 @@ static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
 		 * if current list is non-empty, grab first entry.
 		 */
 		cfqq = list_entry_cfqq(cfqd->cur_rr.next);
-	} else if (!RB_EMPTY_ROOT(&cfqd->service_tree)) {
-		struct rb_node *n = rb_first(&cfqd->service_tree);
+	} else if (!RB_EMPTY_ROOT(&cfqd->service_tree.rb)) {
+		struct rb_node *n = cfq_rb_first(&cfqd->service_tree);
 
 		cfqq = rb_entry(n, struct cfq_queue, rb_node);
 	} else if (!list_empty(&cfqd->idle_rr)) {
@@ -1036,7 +1070,7 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
 	int dispatched = 0;
 	struct rb_node *n;
 
-	while ((n = rb_first(&cfqd->service_tree)) != NULL) {
+	while ((n = cfq_rb_first(&cfqd->service_tree)) != NULL) {
 		struct cfq_queue *cfqq = rb_entry(n, struct cfq_queue, rb_node);
 
 		dispatched += __cfq_forced_dispatch_cfqq(cfqq);
@@ -2014,7 +2048,7 @@ static void *cfq_init_queue(request_queue_t *q)
 
 	memset(cfqd, 0, sizeof(*cfqd));
 
-	cfqd->service_tree = RB_ROOT;
+	cfqd->service_tree = CFQ_RB_ROOT;
 	INIT_LIST_HEAD(&cfqd->cur_rr);
 	INIT_LIST_HEAD(&cfqd->idle_rr);
 	INIT_LIST_HEAD(&cfqd->cic_list);