[PATCH] net: ip/tnl: Set iph->id only when don't fragment is not set

[Oliver Herms <oliver.peter.herms@gmail.com>]

In IPv4 the identification field ensures that fragments of different datagrams
are not mixed by the receiver. Packets with Don't Fragment (DF) flag set are not
to be fragmented in transit and thus don't need an identification.
Calculating the identification takes significant CPU time.
This patch will increase IP tunneling performance by ~10% unless DF is not set.
However, DF is set by default which is best practice.

Signed-off-by: Oliver Herms <oliver.peter.herms@gmail.com>
---
 net/ipv4/ip_tunnel_core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 1452a97914a0..8636c1e0e7b7 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -73,7 +73,9 @@ void iptunnel_xmit(struct sock *sk, struct rtable *rt, struct sk_buff *skb,
 	iph->daddr	=	dst;
 	iph->saddr	=	src;
 	iph->ttl	=	ttl;
-	__ip_select_ident(net, iph, skb_shinfo(skb)->gso_segs ?: 1);
+
+	if (unlikely((iph->frag_off & htons(IP_DF)) == false))
+		__ip_select_ident(net, iph, skb_shinfo(skb)->gso_segs ?: 1);
 
 	err = ip_local_out(net, sk, skb);
 
-- 
2.19.1