Re: [PATCH net-next v5 2/2] seg6: skip the route refcount in seg6local IPv4 lookups

[Hangbin Liu]

On Mon, Nov 09, 2026 at 09:10:15AM +0900, Yuya Kusakabe wrote:
> 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 <yuya.kusakabe@xxxxxxxxx>
> ---
>  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
> 

Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>