net: sock_rps_record_flow() is for connected sockets

Paolo noticed a cache line miss in UDP recvmsg() to access
sk_rxhash, sharing a cache line with sk_drops.

sk_drops might be heavily incremented by cpus handling a flood targeting
this socket.

We might place sk_drops on a separate cache line, but lets try
to avoid wasting 64 bytes per socket just for this, since we have
other bottlenecks to take care of.

sock_rps_record_flow() should only access sk_rxhash for connected
flows.

Testing sk_state for TCP_ESTABLISHED covers most of the cases for
connected sockets, for a zero cost, since system calls using
sock_rps_record_flow() also access sk->sk_prot which is on the
same cache line.

A follow up patch will provide a static_key (Jump Label) since most
hosts do not even use RFS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/sock.h b/include/net/sock.h
index 6dfe3aa22b970eecfab4d4a0753804b1cc82a200..1749e38d03014558ac882b5d1fb37b11ac5e6705 100644 (file)
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -913,7 +913,17 @@ static inline void sock_rps_record_flow_hash(__u32 hash)
 static inline void sock_rps_record_flow(const struct sock *sk)
 {
 #ifdef CONFIG_RPS
-       sock_rps_record_flow_hash(sk->sk_rxhash);
+       /* Reading sk->sk_rxhash might incur an expensive cache line miss.
+        *
+        * TCP_ESTABLISHED does cover almost all states where RFS
+        * might be useful, and is cheaper [1] than testing :
+        *      IPv4: inet_sk(sk)->inet_daddr
+        *      IPv6: ipv6_addr_any(&sk->sk_v6_daddr)
+        * OR   an additional socket flag
+        * [1] : sk_state and sk_prot are in the same cache line.
+        */
+       if (sk->sk_state == TCP_ESTABLISHED)
+               sock_rps_record_flow_hash(sk->sk_rxhash);
 #endif
 }