From 5b8e2f61b9df529ca4af057daf7bfb1de348bdf1 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 6 Dec 2016 19:32:50 -0800 Subject: [PATCH] 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 Reported-by: Paolo Abeni Acked-by: Paolo Abeni Signed-off-by: David S. Miller --- include/net/sock.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/include/net/sock.h b/include/net/sock.h index 6dfe3aa22b97..1749e38d0301 100644 --- 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 } -- 2.20.1