fib_trie: Optimize fib_table_lookup to avoid wasting time on loops/variables

This patch is meant to reduce the complexity of fib_table_lookup by reducing
the number of variables to the bare minimum while still keeping the same if
not improved functionality versus the original.

Most of this change was started off by the desire to rid the function of
chopped_off and current_prefix_length as they actually added very little to
the function since they only applied when computing the cindex.  I was able
to replace them mostly with just a check for the prefix match.  As long as
the prefix between the key and the node being tested was the same we know
we can search the tnode fully versus just testing cindex 0.

The second portion of the change ended up being a massive reordering.
Originally the calls to check_leaf were up near the start of the loop, and
the backtracing and descending into lower levels of tnodes was later.  This
didn't make much sense as the structure of the tree means the leaves are
always the last thing to be tested.  As such I reordered things so that we
instead have a loop that will delve into the tree and only exit when we
have either found a leaf or we have exhausted the tree.  The advantage of
rearranging things like this is that we can fully inline check_leaf since
there is now only one reference to it in the function.
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -90,6 +90,9 @@ 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))
+
 struct tnode {
 	t_key key;
 	unsigned char bits;		/* 2log(KEYLENGTH) bits needed */
@@ -1281,7 +1284,7 @@ static int check_leaf(struct fib_table *tb, struct trie *t, struct tnode *l,
 				continue;
 			fib_alias_accessed(fa);
 			err = fib_props[fa->fa_type].error;
-			if (err) {
+			if (unlikely(err < 0)) {
 #ifdef CONFIG_IP_FIB_TRIE_STATS
 				this_cpu_inc(t->stats->semantic_match_passed);
 #endif
@@ -1303,7 +1306,7 @@ static int check_leaf(struct fib_table *tb, struct trie *t, struct tnode *l,
 				res->prefixlen = li->plen;
 				res->nh_sel = nhsel;
 				res->type = fa->fa_type;
-				res->scope = fa->fa_info->fib_scope;
+				res->scope = fi->fib_scope;
 				res->fi = fi;
 				res->table = tb;
 				res->fa_head = &li->falh;
@@ -1321,23 +1324,24 @@ static int check_leaf(struct fib_table *tb, struct trie *t, struct tnode *l,
 	return 1;
 }
 
+static inline t_key prefix_mismatch(t_key key, struct tnode *n)
+{
+	t_key prefix = n->key;
+
+	return (key ^ prefix) & (prefix | -prefix);
+}
+
 int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
 		     struct fib_result *res, int fib_flags)
 {
-	struct trie *t = (struct trie *) tb->tb_data;
+	struct trie *t = (struct trie *)tb->tb_data;
 #ifdef CONFIG_IP_FIB_TRIE_STATS
 	struct trie_use_stats __percpu *stats = t->stats;
 #endif
-	int ret;
-	struct tnode *n;
-	struct tnode *pn;
-	unsigned int pos, bits;
-	t_key key = ntohl(flp->daddr);
-	unsigned int chopped_off;
-	t_key cindex = 0;
-	unsigned int current_prefix_length = KEYLENGTH;
-	struct tnode *cn;
-	t_key pref_mismatch;
+	const t_key key = ntohl(flp->daddr);
+	struct tnode *n, *pn;
+	t_key cindex;
+	int ret = 1;
 
 	rcu_read_lock();
 
@@ -1349,170 +1353,102 @@ int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
 	this_cpu_inc(stats->gets);
 #endif
 
-	/* Just a leaf? */
-	if (IS_LEAF(n)) {
-		ret = check_leaf(tb, t, n, key, flp, res, fib_flags);
-		goto found;
-	}
-
 	pn = n;
-	chopped_off = 0;
-
-	while (pn) {
-		pos = pn->pos;
-		bits = pn->bits;
-
-		if (!chopped_off)
-			cindex = tkey_extract_bits(mask_pfx(key, current_prefix_length),
-						   pos, bits);
-
-		n = tnode_get_child_rcu(pn, cindex);
-
-		if (n == NULL) {
-#ifdef CONFIG_IP_FIB_TRIE_STATS
-			this_cpu_inc(stats->null_node_hit);
-#endif
-			goto backtrace;
-		}
+	cindex = 0;
+
+	/* Step 1: Travel to the longest prefix match in the trie */
+	for (;;) {
+		unsigned long index = get_index(key, n);
+
+		/* This bit of code is a bit tricky but it combines multiple
+		 * checks into a single check.  The prefix consists of the
+		 * prefix plus zeros for the "bits" in the prefix. The index
+		 * is the difference between the key and this value.  From
+		 * this we can actually derive several pieces of data.
+		 *   if !(index >> bits)
+		 *     we know the value is child index
+		 *   else
+		 *     we have a mismatch in skip bits and failed
+		 */
+		if (index >> n->bits)
+			break;
 
-		if (IS_LEAF(n)) {
-			ret = check_leaf(tb, t, n, key, flp, res, fib_flags);
-			if (ret > 0)
-				goto backtrace;
+		/* we have found a leaf. Prefixes have already been compared */
+		if (IS_LEAF(n))
 			goto found;
-		}
-
-		cn = n;
 
-		/*
-		 * It's a tnode, and we can do some extra checks here if we
-		 * like, to avoid descending into a dead-end branch.
-		 * This tnode is in the parent's child array at index
-		 * key[p_pos..p_pos+p_bits] but potentially with some bits
-		 * chopped off, so in reality the index may be just a
-		 * subprefix, padded with zero at the end.
-		 * We can also take a look at any skipped bits in this
-		 * tnode - everything up to p_pos is supposed to be ok,
-		 * and the non-chopped bits of the index (se previous
-		 * paragraph) are also guaranteed ok, but the rest is
-		 * considered unknown.
-		 *
-		 * The skipped bits are key[pos+bits..cn->pos].
+		/* only record pn and cindex if we are going to be chopping
+		 * bits later.  Otherwise we are just wasting cycles.
 		 */
+		if (index) {
+			pn = n;
+			cindex = index;
+		}
 
-		/* If current_prefix_length < pos+bits, we are already doing
-		 * actual prefix  matching, which means everything from
-		 * pos+(bits-chopped_off) onward must be zero along some
-		 * branch of this subtree - otherwise there is *no* valid
-		 * prefix present. Here we can only check the skipped
-		 * bits. Remember, since we have already indexed into the
-		 * parent's child array, we know that the bits we chopped of
-		 * *are* zero.
-		 */
-
-		/* NOTA BENE: Checking only skipped bits
-		   for the new node here */
-
-		if (current_prefix_length < pos+bits) {
-			if (tkey_extract_bits(cn->key, current_prefix_length,
-						cn->pos - current_prefix_length)
-			    || !(cn->child[0]))
+		n = rcu_dereference(n->child[index]);
+		if (unlikely(!n))
 			goto backtrace;
 	}
 
-		/*
-		 * If chopped_off=0, the index is fully validated and we
-		 * only need to look at the skipped bits for this, the new,
-		 * tnode. What we actually want to do is to find out if
-		 * these skipped bits match our key perfectly, or if we will
-		 * have to count on finding a matching prefix further down,
-		 * because if we do, we would like to have some way of
-		 * verifying the existence of such a prefix at this point.
-		 */
-
-		/* The only thing we can do at this point is to verify that
-		 * any such matching prefix can indeed be a prefix to our
-		 * key, and if the bits in the node we are inspecting that
-		 * do not match our key are not ZERO, this cannot be true.
-		 * Thus, find out where there is a mismatch (before cn->pos)
-		 * and verify that all the mismatching bits are zero in the
-		 * new tnode's key.
-		 */
-
-		/*
-		 * Note: We aren't very concerned about the piece of
-		 * the key that precede pn->pos+pn->bits, since these
-		 * have already been checked. The bits after cn->pos
-		 * aren't checked since these are by definition
-		 * "unknown" at this point. Thus, what we want to see
-		 * is if we are about to enter the "prefix matching"
-		 * state, and in that case verify that the skipped
-		 * bits that will prevail throughout this subtree are
-		 * zero, as they have to be if we are to find a
-		 * matching prefix.
-		 */
+	/* Step 2: Sort out leaves and begin backtracing for longest prefix */
+	for (;;) {
+		/* record the pointer where our next node pointer is stored */
+		struct tnode __rcu **cptr = n->child;
 
-		pref_mismatch = mask_pfx(cn->key ^ key, cn->pos);
-
-		/*
-		 * In short: If skipped bits in this node do not match
-		 * the search key, enter the "prefix matching"
-		 * state.directly.
+		/* This test verifies that none of the bits that differ
+		 * between the key and the prefix exist in the region of
+		 * the lsb and higher in the prefix.
 		 */
-		if (pref_mismatch) {
-			/* fls(x) = __fls(x) + 1 */
-			int mp = KEYLENGTH - __fls(pref_mismatch) - 1;
-
-			if (tkey_extract_bits(cn->key, mp, cn->pos - mp) != 0)
+		if (unlikely(prefix_mismatch(key, n)))
 			goto backtrace;
 
-			if (current_prefix_length >= cn->pos)
-				current_prefix_length = mp;
-		}
+		/* exit out and process leaf */
+		if (unlikely(IS_LEAF(n)))
+			break;
 
-		pn = n; /* Descend */
-		chopped_off = 0;
-		continue;
+		/* Don't bother recording parent info.  Since we are in
+		 * prefix match mode we will have to come back to wherever
+		 * we started this traversal anyway
+		 */
 
+		while ((n = rcu_dereference(*cptr)) == NULL) {
 backtrace:
-		chopped_off++;
-
-		/* As zero don't change the child key (cindex) */
-		while ((chopped_off <= pn->bits)
-		       && !(cindex & (1<<(chopped_off-1))))
-			chopped_off++;
-
-		/* Decrease current_... with bits chopped off */
-		if (current_prefix_length > pn->pos + pn->bits - chopped_off)
-			current_prefix_length = pn->pos + pn->bits
-				- chopped_off;
-
-		/*
-		 * Either we do the actual chop off according or if we have
-		 * chopped off all bits in this tnode walk up to our parent.
+#ifdef CONFIG_IP_FIB_TRIE_STATS
+			if (!n)
+				this_cpu_inc(stats->null_node_hit);
+#endif
+			/* If we are at cindex 0 there are no more bits for
+			 * us to strip at this level so we must ascend back
+			 * up one level to see if there are any more bits to
+			 * be stripped there.
 			 */
+			while (!cindex) {
+				t_key pkey = pn->key;
 
-		if (chopped_off <= pn->bits) {
-			cindex &= ~(1 << (chopped_off-1));
-		} else {
-			struct tnode *parent = node_parent_rcu(pn);
-			if (!parent)
+				pn = node_parent_rcu(pn);
+				if (unlikely(!pn))
 					goto failed;
-
-			/* Get Child's index */
-			cindex = tkey_extract_bits(pn->key, parent->pos, parent->bits);
-			pn = parent;
-			chopped_off = 0;
-
 #ifdef CONFIG_IP_FIB_TRIE_STATS
 				this_cpu_inc(stats->backtrack);
 #endif
-			goto backtrace;
+				/* Get Child's index */
+				cindex = get_index(pkey, pn);
 			}
+
+			/* strip the least significant bit from the cindex */
+			cindex &= cindex - 1;
+
+			/* grab pointer for next child node */
+			cptr = &pn->child[cindex];
 		}
-failed:
-	ret = 1;
+	}
+
 found:
+	/* Step 3: Process the leaf, if that fails fall back to backtracing */
+	ret = check_leaf(tb, t, n, key, flp, res, fib_flags);
+	if (unlikely(ret > 0))
+		goto backtrace;
+failed:
 	rcu_read_unlock();
 	return ret;
 }