[PATCH net-next] sock: avoid dirtying incoming_cpu if not needed

[PATCH net-next] sock: avoid dirtying incoming_cpu if not needed

[Paolo Abeni]

for connected socket, the incoming_cpu field in the sock struct
is not going to change frequently, but we are setting it
unconditionally for each packet.

Since sk_incoming_cpu and sk_flags share the same cacheline,
and the latter is access by udp_recvmsg(), this cause a cache
miss for each packet for UDP connected socket.

With this patch, we set the incoming cpu field only when the
ingress cpu really changes.

This gives a small but measurable performance improvement for
connected UDP socket.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 include/net/sock.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 858891c..00d0914 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -907,7 +907,10 @@ static inline int sk_backlog_rcv(struct sock *sk, struct sk_buff *skb)
 
 static inline void sk_incoming_cpu_update(struct sock *sk)
 {
-	sk->sk_incoming_cpu = raw_smp_processor_id();
+	int cpu = raw_smp_processor_id();
+
+	if (unlikely(sk->sk_incoming_cpu != cpu))
+		sk->sk_incoming_cpu = cpu;
 }
 
 static inline void sock_rps_record_flow_hash(__u32 hash)
-- 
2.9.4

Re: [PATCH net-next] sock: avoid dirtying incoming_cpu if not needed

[David Miller]

From: Paolo Abeni <pabeni@redhat.com>
Date: Wed, 21 Jun 2017 11:45:31 +0200

> for connected socket, the incoming_cpu field in the sock struct
> is not going to change frequently, but we are setting it
> unconditionally for each packet.
> 
> Since sk_incoming_cpu and sk_flags share the same cacheline,
> and the latter is access by udp_recvmsg(), this cause a cache
> miss for each packet for UDP connected socket.
> 
> With this patch, we set the incoming cpu field only when the
> ingress cpu really changes.
> 
> This gives a small but measurable performance improvement for
> connected UDP socket.
> 
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied.