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>
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
}