fib_trie: Update meaning of pos to represent unchecked bits

This change moves the pos value to the other side of the "bits" field.  By
doing this it actually simplifies a significant amount of code in the trie.

For example when halving a tree we know that the bit lost exists at
oldnode->pos, and if we inflate the tree the new bit being add is at
tn->pos.  Previously to find those bits you would have to subtract pos and
bits from the keylength or start with a value of (1 << 31) and then shift
that.

There are a number of spots throughout the code that benefit from this.  In
the case of the hot-path searches the main advantage is that we can drop 2
or more operations from the search path as we no longer need to compute the
value for the index to be shifted by and can instead just use the raw pos
value.

In addition the tkey_extract_bits is now defunct and can be replaced by
get_index since the two operations were doing the same thing, but now
get_index does it much more quickly as it is only an xor and shift versus a
pair of shifts and a subtraction.
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -90,8 +90,7 @@ typedef unsigned int t_key;
 #define IS_TNODE(n) ((n)->bits)
 #define IS_LEAF(n) (!(n)->bits)
 
-#define get_shift(_kv) (KEYLENGTH - (_kv)->pos - (_kv)->bits)
-#define get_index(_key, _kv) (((_key) ^ (_kv)->key) >> get_shift(_kv))
+#define get_index(_key, _kv) (((_key) ^ (_kv)->key) >> (_kv)->pos)
 
 struct tnode {
 	t_key key;
@@ -209,81 +208,64 @@ static inline struct tnode *tnode_get_child_rcu(const struct tnode *tn, unsigned
 	return rcu_dereference_rtnl(tn->child[i]);
 }
 
-static inline t_key mask_pfx(t_key k, unsigned int l)
-{
-	return (l == 0) ? 0 : k >> (KEYLENGTH-l) << (KEYLENGTH-l);
-}
-
-static inline t_key tkey_extract_bits(t_key a, unsigned int offset, unsigned int bits)
-{
-	if (offset < KEYLENGTH)
-		return ((t_key)(a << offset)) >> (KEYLENGTH - bits);
-	else
-		return 0;
-}
-
-/*
-  To understand this stuff, an understanding of keys and all their bits is
-  necessary. Every node in the trie has a key associated with it, but not
-  all of the bits in that key are significant.
-
-  Consider a node 'n' and its parent 'tp'.
-
-  If n is a leaf, every bit in its key is significant. Its presence is
-  necessitated by path compression, since during a tree traversal (when
-  searching for a leaf - unless we are doing an insertion) we will completely
-  ignore all skipped bits we encounter. Thus we need to verify, at the end of
-  a potentially successful search, that we have indeed been walking the
-  correct key path.
-
-  Note that we can never "miss" the correct key in the tree if present by
-  following the wrong path. Path compression ensures that segments of the key
-  that are the same for all keys with a given prefix are skipped, but the
-  skipped part *is* identical for each node in the subtrie below the skipped
-  bit! trie_insert() in this implementation takes care of that - note the
-  call to tkey_sub_equals() in trie_insert().
-
-  if n is an internal node - a 'tnode' here, the various parts of its key
-  have many different meanings.
-
-  Example:
-  _________________________________________________________________
-  | i | i | i | i | i | i | i | N | N | N | S | S | S | S | S | C |
-  -----------------------------------------------------------------
-    0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
-
-  _________________________________________________________________
-  | C | C | C | u | u | u | u | u | u | u | u | u | u | u | u | u |
-  -----------------------------------------------------------------
-   16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31
-
-  tp->pos = 7
-  tp->bits = 3
-  n->pos = 15
-  n->bits = 4
-
-  First, let's just ignore the bits that come before the parent tp, that is
-  the bits from 0 to (tp->pos-1). They are *known* but at this point we do
-  not use them for anything.
-
-  The bits from (tp->pos) to (tp->pos + tp->bits - 1) - "N", above - are the
-  index into the parent's child array. That is, they will be used to find
-  'n' among tp's children.
-
-  The bits from (tp->pos + tp->bits) to (n->pos - 1) - "S" - are skipped bits
-  for the node n.
-
-  All the bits we have seen so far are significant to the node n. The rest
-  of the bits are really not needed or indeed known in n->key.
-
-  The bits from (n->pos) to (n->pos + n->bits - 1) - "C" - are the index into
-  n's child array, and will of course be different for each child.
-
-
-  The rest of the bits, from (n->pos + n->bits) onward, are completely unknown
-  at this point.
-
-*/
+/* To understand this stuff, an understanding of keys and all their bits is
+ * necessary. Every node in the trie has a key associated with it, but not
+ * all of the bits in that key are significant.
+ *
+ * Consider a node 'n' and its parent 'tp'.
+ *
+ * If n is a leaf, every bit in its key is significant. Its presence is
+ * necessitated by path compression, since during a tree traversal (when
+ * searching for a leaf - unless we are doing an insertion) we will completely
+ * ignore all skipped bits we encounter. Thus we need to verify, at the end of
+ * a potentially successful search, that we have indeed been walking the
+ * correct key path.
+ *
+ * Note that we can never "miss" the correct key in the tree if present by
+ * following the wrong path. Path compression ensures that segments of the key
+ * that are the same for all keys with a given prefix are skipped, but the
+ * skipped part *is* identical for each node in the subtrie below the skipped
+ * bit! trie_insert() in this implementation takes care of that.
+ *
+ * if n is an internal node - a 'tnode' here, the various parts of its key
+ * have many different meanings.
+ *
+ * Example:
+ * _________________________________________________________________
+ * | i | i | i | i | i | i | i | N | N | N | S | S | S | S | S | C |
+ * -----------------------------------------------------------------
+ *  31  30  29  28  27  26  25  24  23  22  21  20  19  18  17  16
+ *
+ * _________________________________________________________________
+ * | C | C | C | u | u | u | u | u | u | u | u | u | u | u | u | u |
+ * -----------------------------------------------------------------
+ *  15  14  13  12  11  10   9   8   7   6   5   4   3   2   1   0
+ *
+ * tp->pos = 22
+ * tp->bits = 3
+ * n->pos = 13
+ * n->bits = 4
+ *
+ * First, let's just ignore the bits that come before the parent tp, that is
+ * the bits from (tp->pos + tp->bits) to 31. They are *known* but at this
+ * point we do not use them for anything.
+ *
+ * The bits from (tp->pos) to (tp->pos + tp->bits - 1) - "N", above - are the
+ * index into the parent's child array. That is, they will be used to find
+ * 'n' among tp's children.
+ *
+ * The bits from (n->pos + n->bits) to (tn->pos - 1) - "S" - are skipped bits
+ * for the node n.
+ *
+ * All the bits we have seen so far are significant to the node n. The rest
+ * of the bits are really not needed or indeed known in n->key.
+ *
+ * The bits from (n->pos) to (n->pos + n->bits - 1) - "C" - are the index into
+ * n's child array, and will of course be different for each child.
+ *
+ * The rest of the bits, from 0 to (n->pos + n->bits), are completely unknown
+ * at this point.
+ */
 
 static const int halve_threshold = 25;
 static const int inflate_threshold = 50;
@@ -367,7 +349,7 @@ static struct tnode *leaf_new(t_key key)
 		 * as the nodes are searched
 		 */
 		l->key = key;
-		l->pos = KEYLENGTH;
+		l->pos = 0;
 		/* set bits to 0 indicating we are not a tnode */
 		l->bits = 0;
 
@@ -400,7 +382,7 @@ static struct tnode *tnode_new(t_key key, int pos, int bits)
 		tn->parent = NULL;
 		tn->pos = pos;
 		tn->bits = bits;
-		tn->key = mask_pfx(key, pos);
+		tn->key = (shift < KEYLENGTH) ? (key >> shift) << shift : 0;
 		tn->full_children = 0;
 		tn->empty_children = 1<<bits;
 	}
@@ -410,14 +392,12 @@ static struct tnode *tnode_new(t_key key, int pos, int bits)
 	return tn;
 }
 
-/*
- * Check whether a tnode 'n' is "full", i.e. it is an internal node
+/* Check whether a tnode 'n' is "full", i.e. it is an internal node
  * and no bits are skipped. See discussion in dyntree paper p. 6
  */
-
 static inline int tnode_full(const struct tnode *tn, const struct tnode *n)
 {
-	return n && IS_TNODE(n) && (n->pos == (tn->pos + tn->bits));
+	return n && ((n->pos + n->bits) == tn->pos) && IS_TNODE(n);
 }
 
 static inline void put_child(struct tnode *tn, int i,
@@ -641,11 +621,12 @@ static struct tnode *inflate(struct trie *t, struct tnode *oldtnode)
 {
 	int olen = tnode_child_length(oldtnode);
 	struct tnode *tn;
+	t_key m;
 	int i;
 
 	pr_debug("In inflate\n");
 
-	tn = tnode_new(oldtnode->key, oldtnode->pos, oldtnode->bits + 1);
+	tn = tnode_new(oldtnode->key, oldtnode->pos - 1, oldtnode->bits + 1);
 
 	if (!tn)
 		return ERR_PTR(-ENOMEM);
@@ -656,21 +637,18 @@ static struct tnode *inflate(struct trie *t, struct tnode *oldtnode)
 	 * fails. In case of failure we return the oldnode and  inflate
 	 * of tnode is ignored.
 	 */
+	for (i = 0, m = 1u << tn->pos; i < olen; i++) {
+		struct tnode *inode = tnode_get_child(oldtnode, i);
 
-	for (i = 0; i < olen; i++) {
-		struct tnode *inode;
-
-		inode = tnode_get_child(oldtnode, i);
-		if (tnode_full(oldtnode, inode) && inode->bits > 1) {
+		if (tnode_full(oldtnode, inode) && (inode->bits > 1)) {
 			struct tnode *left, *right;
-			t_key m = ~0U << (KEYLENGTH - 1) >> inode->pos;
 
-			left = tnode_new(inode->key&(~m), inode->pos + 1,
+			left = tnode_new(inode->key & ~m, inode->pos,
 					 inode->bits - 1);
 			if (!left)
 				goto nomem;
 
-			right = tnode_new(inode->key|m, inode->pos + 1,
+			right = tnode_new(inode->key | m, inode->pos,
 					  inode->bits - 1);
 
 			if (!right) {
@@ -694,9 +672,7 @@ static struct tnode *inflate(struct trie *t, struct tnode *oldtnode)
 
 		/* A leaf or an internal node with skipped bits */
 		if (!tnode_full(oldtnode, inode)) {
-			put_child(tn,
-				tkey_extract_bits(inode->key, tn->pos, tn->bits),
-				inode);
+			put_child(tn, get_index(inode->key, tn), inode);
 			continue;
 		}
 
@@ -767,7 +743,7 @@ static struct tnode *halve(struct trie *t, struct tnode *oldtnode)
 
 	pr_debug("In halve\n");
 
-	tn = tnode_new(oldtnode->key, oldtnode->pos, oldtnode->bits - 1);
+	tn = tnode_new(oldtnode->key, oldtnode->pos + 1, oldtnode->bits - 1);
 
 	if (!tn)
 		return ERR_PTR(-ENOMEM);
@@ -787,7 +763,7 @@ static struct tnode *halve(struct trie *t, struct tnode *oldtnode)
 		if (left && right) {
 			struct tnode *newn;
 
-			newn = tnode_new(left->key, tn->pos + tn->bits, 1);
+			newn = tnode_new(left->key, oldtnode->pos, 1);
 
 			if (!newn)
 				goto nomem;
@@ -915,7 +891,7 @@ static void trie_rebalance(struct trie *t, struct tnode *tn)
 	key = tn->key;
 
 	while (tn != NULL && (tp = node_parent(tn)) != NULL) {
-		cindex = tkey_extract_bits(key, tp->pos, tp->bits);
+		cindex = get_index(key, tp);
 		wasfull = tnode_full(tp, tnode_get_child(tp, cindex));
 		tn = resize(t, tn);
 
@@ -1005,11 +981,8 @@ static struct list_head *fib_insert_node(struct trie *t, u32 key, int plen)
 	 */
 	if (n) {
 		struct tnode *tn;
-		int newpos;
-
-		newpos = KEYLENGTH - __fls(n->key ^ key) - 1;
 
-		tn = tnode_new(key, newpos, 1);
+		tn = tnode_new(key, __fls(key ^ n->key), 1);
 		if (!tn) {
 			free_leaf_info(li);
 			node_free(l);
@@ -1559,12 +1532,7 @@ static int trie_flush_leaf(struct tnode *l)
 static struct tnode *leaf_walk_rcu(struct tnode *p, struct tnode *c)
 {
 	do {
-		t_key idx;
-
-		if (c)
-			idx = tkey_extract_bits(c->key, p->pos, p->bits) + 1;
-		else
-			idx = 0;
+		t_key idx = c ? idx = get_index(c->key, p) + 1 : 0;
 
 		while (idx < 1u << p->bits) {
 			c = tnode_get_child_rcu(p, idx++);
@@ -1851,7 +1819,7 @@ static struct tnode *fib_trie_get_next(struct fib_trie_iter *iter)
 	/* Current node exhausted, pop back up */
 	p = node_parent_rcu(tn);
 	if (p) {
-		cindex = tkey_extract_bits(tn->key, p->pos, p->bits)+1;
+		cindex = get_index(tn->key, p) + 1;
 		tn = p;
 		--iter->depth;
 		goto rescan;
@@ -2186,10 +2154,10 @@ static int fib_trie_seq_show(struct seq_file *seq, void *v)
 	if (IS_TNODE(n)) {
 		__be32 prf = htonl(n->key);
 
-		seq_indent(seq, iter->depth - 1);
-		seq_printf(seq, "  +-- %pI4/%d %d %d %d\n",
-			   &prf, n->pos, n->bits, n->full_children,
-			   n->empty_children);
+		seq_indent(seq, iter->depth-1);
+		seq_printf(seq, "  +-- %pI4/%zu %u %u %u\n",
+			   &prf, KEYLENGTH - n->pos - n->bits, n->bits,
+			   n->full_children, n->empty_children);
 	} else {
 		struct leaf_info *li;
 		__be32 val = htonl(n->key);