net: ipv4 only populate IP_PKTINFO when needed

The since the removal of the routing cache computing
fib_compute_spec_dst() does a fib_table lookup for each UDP multicast
packet received.  This has introduced a performance regression for some
UDP workloads.

This change skips populating the packet info for sockets that do not have
IP_PKTINFO set.

Benchmark results from a netperf UDP_RR test:
Before 89789.68 transactions/s
After  90587.62 transactions/s

Benchmark results from a fio 1 byte UDP multicast pingpong test
(Multicast one way unicast response):
Before 12.63us RTT
After  12.48us RTT

Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 56e34457ac07..0626f2cb192e 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1052,11 +1052,12 @@ e_inval:
  * destination in skb->cb[] before dst drop.
  * This way, receiver doesnt make cache line misses to read rtable.
  */
-void ipv4_pktinfo_prepare(struct sk_buff *skb)
+void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
 {
         struct in_pktinfo *pktinfo = PKTINFO_SKB_CB(skb);
 
-        if (skb_rtable(skb)) {
+        if ((inet_sk(sk)->cmsg_flags & IP_CMSG_PKTINFO) &&
+            skb_rtable(skb)) {
                 pktinfo->ipi_ifindex = inet_iif(skb);
                 pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
         } else {