ipv4/udp: Add 4-tuple hash for connected socket

Currently, the udp_table has two hash table, the port hash and portaddr
hash. Usually for UDP servers, all sockets have the same local port and
addr, so they are all on the same hash slot within a reuseport group.

In some applications, UDP servers use connect() to manage clients. In
particular, when firstly receiving from an unseen 4 tuple, a new socket
is created and connect()ed to the remote addr:port, and then the fd is
used exclusively by the client.

Once there are connected sks in a reuseport group, udp has to score all
sks in the same hash2 slot to find the best match. This could be
inefficient with a large number of connections, resulting in high
softirq overhead.

To solve the problem, this patch implement 4-tuple hash for connected
udp sockets. During connect(), hash4 slot is updated, as well as a
corresponding counter, hash4_cnt, in hslot2. In __udp4_lib_lookup(),
hslot4 will be searched firstly if the counter is non-zero. Otherwise,
hslot2 is used like before. Note that only connected sockets enter this
hash4 path, while un-connected ones are not affected.

hlist_nulls is used for hash4, because we probably move to another hslot
wrongly when lookup with concurrent rehash. Then we check nulls at the
list end to see if we should restart lookup. Because udp does not use
SLAB_TYPESAFE_BY_RCU, we don't need to touch sk_refcnt when lookup.

Stress test results (with 1 cpu fully used) are shown below, in pps:
(1) _un-connected_ socket as server
    [a] w/o hash4: 1,825176
    [b] w/  hash4: 1,831750 (+0.36%)

(2) 500 _connected_ sockets as server
    [c] w/o hash4:   290860 (only 16% of [a])
    [d] w/  hash4: 1,889658 (+3.1% compared with [b])

With hash4, compute_score is skipped when lookup, so [d] is slightly
better than [b].

Co-developed-by: Cambda Zhu <cambda@linux.alibaba.com>
Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
Co-developed-by: Fred Chen <fred.cc@alibaba-inc.com>
Signed-off-by: Fred Chen <fred.cc@alibaba-inc.com>
Co-developed-by: Yubing Qiu <yubing.qiuyubing@alibaba-inc.com>
Signed-off-by: Yubing Qiu <yubing.qiuyubing@alibaba-inc.com>
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 1 deletions

diff --git a/include/net/udp.h b/include/net/udp.h
index edb669967130..feb06c0e48fb 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -302,13 +302,27 @@ static inline int udp_lib_hash(struct sock *sk)
 }
 
 void udp_lib_unhash(struct sock *sk);
-void udp_lib_rehash(struct sock *sk, u16 new_hash);
+void udp_lib_rehash(struct sock *sk, u16 new_hash, u16 new_hash4);
 
 static inline void udp_lib_close(struct sock *sk, long timeout)
 {
 	sk_common_release(sk);
 }
 
+/* hash4 routines shared between UDPv4/6 */
+#if IS_ENABLED(CONFIG_BASE_SMALL)
+static inline void udp_lib_hash4(struct sock *sk, u16 hash)
+{
+}
+
+static inline void udp4_hash4(struct sock *sk)
+{
+}
+#else /* !CONFIG_BASE_SMALL */
+void udp_lib_hash4(struct sock *sk, u16 hash);
+void udp4_hash4(struct sock *sk);
+#endif /* CONFIG_BASE_SMALL */
+
 int udp_lib_get_port(struct sock *sk, unsigned short snum,
 		     unsigned int hash2_nulladdr);