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
Cc: Vlad Dumitrescu
Cc: Alexei Starovoitov
Cc: Daniel Borkmann
---
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
