[PATCH net-next v4 2/2] seg6: skip the route refcount in seg6local IPv4 lookups

ip_route_input() looks the input route up with ip_route_input_noref()
and then forces a reference on it, which is dropped again when the
packet is transmitted.  End.DX4 and End.DT4 call it and hand the skb
to dst_input() within one RCU read-side section: the one of the
receive path, or that of the nfnetlink verdict handler for a packet
reinjected from NFQUEUE.

Use ip_route_input_noref(), as ip_rcv_finish() does.

Throughput at 0.5% packet loss of 82-byte IPv4-in-IPv6 UDP packets
(no SRH), spread by RSS on an ixgbe 82599ES across two 10-core Xeon
E5-2650 v3, offered by TRex over 10 runs of 10 s, in Mpps:

  End.DX4
  queues  cores                   before   after
  1       1                        0.791   0.811  (+2.5%)
  10      10 on the NIC's socket   6.118   7.394  (+20.9%)
  16      10 + 6 on two sockets    3.305  10.205  (+208.8%)

  End.DT4
  queues  cores                   before   after
  1       1                        0.481   0.489  (+1.6%)
  10      10 on the NIC's socket   4.328   4.588  (+6.0%)
  16      10 + 6 on two sockets    3.351   7.153  (+113.5%)

Every CPU takes and drops a reference on the same input route, the
one cached on the nexthop.  At 16 queues that cache line bounces
between the sockets, and skb_dst_force() and dst_release() together
take more than half of the cycles for End.DX4 and over 40% for End.DT4.

Signed-off-by: Yuya Kusakabe <redacted>
---
 net/ipv6/seg6_local.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 7fbcf590297d..209e31c21e33 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c

@@ -1096,7 +1096,7 @@ static int input_action_end_dx4_finish(struct net *net, struct sock *sk,
 
 	skb_dst_drop(skb);
 
-	reason = ip_route_input(skb, nhaddr, iph->saddr, 0, skb->dev);
+	reason = ip_route_input_noref(skb, nhaddr, iph->saddr, 0, skb->dev);
 	if (reason) {
 		kfree_skb_reason(skb, reason);
 		return -EINVAL;

@@ -1322,7 +1322,7 @@ static int input_action_end_dt4(struct sk_buff *skb,
 
 	iph = ip_hdr(skb);
 
-	reason = ip_route_input(skb, iph->daddr, iph->saddr, 0, skb->dev);
+	reason = ip_route_input_noref(skb, iph->daddr, iph->saddr, 0, skb->dev);
 	if (unlikely(reason))
 		goto drop;
 

-- 
2.50.1