[PATCH bpf-next v2 1/1] bpf, lpm: make longest_prefix_match() faster

[Eric Dumazet <edumazet@google.com>]

At LPC 2018 in Vancouver, Vlad Dumitrescu mentioned that longest_prefix_match()
has a high cost [1].

One reason for that cost is a loop handling one byte at a time.

We can handle more bytes at a time, if enough attention is paid
to endianness.

I was able to remove ~55 % of longest_prefix_match() cpu costs.

[1] https://linuxplumbersconf.org/event/2/contributions/88/attachments/76/87/lpc-bpf-2018-shaping.pdf

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vlad Dumitrescu <vladum@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
---
v2: fixed Daniel and Alexei email addresses... :/

 kernel/bpf/lpm_trie.c | 59 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 49 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 9058317ba9de2eae4f28e8ca98c3c30bc6167c24..bfd4882e1106c699b62217eef0c2cc92f31077c6 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -168,20 +168,59 @@ static size_t longest_prefix_match(const struct lpm_trie *trie,
 				   const struct lpm_trie_node *node,
 				   const struct bpf_lpm_trie_key *key)
 {
-	size_t prefixlen = 0;
-	size_t i;
+	u32 limit = min(node->prefixlen, key->prefixlen);
+	u32 prefixlen = 0, i = 0;
 
-	for (i = 0; i < trie->data_size; i++) {
-		size_t b;
+	BUILD_BUG_ON(offsetof(struct lpm_trie_node, data) % sizeof(u32));
+	BUILD_BUG_ON(offsetof(struct bpf_lpm_trie_key, data) % sizeof(u32));
 
-		b = 8 - fls(node->data[i] ^ key->data[i]);
-		prefixlen += b;
+#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && defined(CONFIG_64BIT)
 
-		if (prefixlen >= node->prefixlen || prefixlen >= key->prefixlen)
-			return min(node->prefixlen, key->prefixlen);
+	/* data_size >= 16 has very small probability.
+	 * We do not use a loop for optimal code generation.
+	 */
+	if (trie->data_size >= 8) {
+		u64 diff = be64_to_cpu(*(__be64 *)node->data ^
+				       *(__be64 *)key->data);
 
-		if (b < 8)
-			break;
+		prefixlen = 64 - fls64(diff);
+		if (prefixlen >= limit)
+			return limit;
+		if (diff)
+			return prefixlen;
+		i = 8;
+	}
+#endif
+
+	while (trie->data_size >= i + 4) {
+		u32 diff = be32_to_cpu(*(__be32 *)&node->data[i] ^
+				       *(__be32 *)&key->data[i]);
+
+		prefixlen += 32 - fls(diff);
+		if (prefixlen >= limit)
+			return limit;
+		if (diff)
+			return prefixlen;
+		i += 4;
+	}
+
+	if (trie->data_size >= i + 2) {
+		u16 diff = be16_to_cpu(*(__be16 *)&node->data[i] ^
+				       *(__be16 *)&key->data[i]);
+
+		prefixlen += 16 - fls(diff);
+		if (prefixlen >= limit)
+			return limit;
+		if (diff)
+			return prefixlen;
+		i += 2;
+	}
+
+	if (trie->data_size >= i + 1) {
+		prefixlen += 8 - fls(node->data[i] ^ key->data[i]);
+
+		if (prefixlen >= limit)
+			return limit;
 	}
 
 	return prefixlen;
-- 
2.19.1.1215.g8438c0b245-goog