fib_trie: Add functions should_inflate and should_halve

This change pulls the logic for if we should inflate/halve the nodes out
into separate functions.  It also addresses what I believe is a bug where 1
full node is all that is needed to keep a node from ever being halved.

Simple script to reproduce the issue:
	modprobe dummy;	ifconfig dummy0 up
	for i in `seq 0 255`; do ifconfig dummy0:$i 10.0.${i}.1/24 up; done
	ifconfig dummy0:256 10.0.255.33/16 up
	for i in `seq 0 254`; do ifconfig dummy0:$i down; done

Results from /proc/net/fib_triestat
Before:
	Local:
		Aver depth:     3.00
		Max depth:      4
		Leaves:         17
		Prefixes:       18
		Internal nodes: 11
		  1: 8  2: 2  10: 1
		Pointers: 1048
	Null ptrs: 1021
	Total size: 11  kB
After:
	Local:
		Aver depth:     3.41
		Max depth:      5
		Leaves:         17
		Prefixes:       18
		Internal nodes: 12
		  1: 8  2: 3  3: 1
		Pointers: 36
	Null ptrs: 8
	Total size: 3  kB
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -647,34 +647,7 @@ static struct tnode *halve(struct trie *t, struct tnode *oldtnode)
 	return ERR_PTR(-ENOMEM);
 }
 
-#define MAX_WORK 10
-static struct tnode *resize(struct trie *t, struct tnode *tn)
-{
-	struct tnode *old_tn, *n = NULL;
-	int inflate_threshold_use;
-	int halve_threshold_use;
-	int max_work;
-
-	if (!tn)
-		return NULL;
-
-	pr_debug("In tnode_resize %p inflate_threshold=%d threshold=%d\n",
-		 tn, inflate_threshold, halve_threshold);
-
-	/* No children */
-	if (tn->empty_children > (tnode_child_length(tn) - 1))
-		goto no_children;
-
-	/* One child */
-	if (tn->empty_children == (tnode_child_length(tn) - 1))
-		goto one_child;
-	/*
-	 * Double as long as the resulting node has a number of
-	 * nonempty nodes that are above the threshold.
-	 */
-
-	/*
-	 * From "Implementing a dynamic compressed trie" by Stefan Nilsson of
+/* From "Implementing a dynamic compressed trie" by Stefan Nilsson of
  * the Helsinki University of Technology and Matti Tikkanen of Nokia
  * Telecommunications, page 6:
  * "A node is doubled if the ratio of non-empty children to all
@@ -731,23 +704,58 @@ static struct tnode *resize(struct trie *t, struct tnode *tn)
  *    tnode_child_length(tn)
  *
  */
+static bool should_inflate(const struct tnode *tn)
+{
+	unsigned long used = tnode_child_length(tn);
+	unsigned long threshold = used;
 
 	/* Keep root node larger */
+	threshold *= node_parent(tn) ? inflate_threshold :
+				       inflate_threshold_root;
+	used += tn->full_children;
+	used -= tn->empty_children;
 
-	if (!node_parent(tn)) {
-		inflate_threshold_use = inflate_threshold_root;
-		halve_threshold_use = halve_threshold_root;
-	} else {
-		inflate_threshold_use = inflate_threshold;
-		halve_threshold_use = halve_threshold;
-	}
+	return tn->pos && ((50 * used) >= threshold);
+}
 
-	max_work = MAX_WORK;
-	while ((tn->full_children > 0 &&  max_work-- &&
-		50 * (tn->full_children + tnode_child_length(tn)
-		      - tn->empty_children)
-		>= inflate_threshold_use * tnode_child_length(tn))) {
+static bool should_halve(const struct tnode *tn)
+{
+	unsigned long used = tnode_child_length(tn);
+	unsigned long threshold = used;
 
+	/* Keep root node larger */
+	threshold *= node_parent(tn) ? halve_threshold :
+				       halve_threshold_root;
+	used -= tn->empty_children;
+
+	return (tn->bits > 1) && ((100 * used) < threshold);
+}
+
+#define MAX_WORK 10
+static struct tnode *resize(struct trie *t, struct tnode *tn)
+{
+	struct tnode *old_tn, *n = NULL;
+	int max_work;
+
+	if (!tn)
+		return NULL;
+
+	pr_debug("In tnode_resize %p inflate_threshold=%d threshold=%d\n",
+		 tn, inflate_threshold, halve_threshold);
+
+	/* No children */
+	if (tn->empty_children > (tnode_child_length(tn) - 1))
+		goto no_children;
+
+	/* One child */
+	if (tn->empty_children == (tnode_child_length(tn) - 1))
+		goto one_child;
+
+	/* Double as long as the resulting node has a number of
+	 * nonempty nodes that are above the threshold.
+	 */
+	max_work = MAX_WORK;
+	while (should_inflate(tn) && max_work--) {
 		old_tn = tn;
 		tn = inflate(t, tn);
 
@@ -764,16 +772,11 @@ static struct tnode *resize(struct trie *t, struct tnode *tn)
 	if (max_work != MAX_WORK)
 		return tn;
 
-	/*
-	 * Halve as long as the number of empty children in this
+	/* Halve as long as the number of empty children in this
 	 * node is above threshold.
 	 */
-
 	max_work = MAX_WORK;
-	while (tn->bits > 1 &&  max_work-- &&
-	       100 * (tnode_child_length(tn) - tn->empty_children) <
-	       halve_threshold_use * tnode_child_length(tn)) {
-
+	while (should_halve(tn) && max_work--) {
 		old_tn = tn;
 		tn = halve(t, tn);
 		if (IS_ERR(tn)) {