Subject: Re: [PATCH v2 net-next 1/2] udp_tunnel: create a fastpath GRO lookup.

From: Paolo Abeni <[email-masked]>
Date: Sun, 9 Mar 2025 16:55:24 +0100

On 3/8/25 7:37 PM, Willem de Bruijn wrote:
> Paolo Abeni wrote:
>> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
>> index 2c0725583be39..054d4d4a8927f 100644
>> --- a/net/ipv4/udp_offload.c
>> +++ b/net/ipv4/udp_offload.c
>> @@ -12,6 +12,38 @@
>>  #include <net/udp.h>
>>  #include <net/protocol.h>
>>  #include <net/inet_common.h>
>> +#include <net/udp_tunnel.h>
>> +
>> +#if IS_ENABLED(CONFIG_NET_UDP_TUNNEL)
>> +static DEFINE_SPINLOCK(udp_tunnel_gro_lock);
>> +
>> +void udp_tunnel_update_gro_lookup(struct net *net, struct sock *sk, bool add)
>> +{
>> +	bool is_ipv6 = sk->sk_family == AF_INET6;
>> +	struct udp_sock *tup, *up = udp_sk(sk);
>> +	struct udp_tunnel_gro *udp_tunnel_gro;
>> +
>> +	spin_lock(&udp_tunnel_gro_lock);
>> +	udp_tunnel_gro = &net->ipv4.udp_tunnel_gro[is_ipv6];
>> +	if (add)
>> +		hlist_add_head(&up->tunnel_list, &udp_tunnel_gro->list);
>> +	else
>> +		hlist_del_init(&up->tunnel_list);
>> +
>> +	if (udp_tunnel_gro->list.first &&
>> +	    !udp_tunnel_gro->list.first->next) {
>> +		tup = hlist_entry(udp_tunnel_gro->list.first, struct udp_sock,
>> +				  tunnel_list);
>> +
>> +		rcu_assign_pointer(udp_tunnel_gro->sk, (struct sock *)tup);
> 
> If the targeted case is a single tunnel, is it worth maintaining the list?
> 
> If I understand correctly, it is only there to choose a fall-back when the
> current tup is removed. But complicates the code quite a bit.

I'll try to answer the questions on both patches here.

I guess in the end there is a relevant amount of personal preferences.
Overall accounting is ~20 lines, IMHO it's not much.

I think we should at least preserve the optimization when the relevant
tunnel is deleted and re-created, and the minimal accounting required
for that will drop just a bunch of lines from
udp_tunnel_update_gro_lookup(), while keeping all the hooking.

Additionally I think it would be surprising transiently applying some
unusual configuration and as a side effect get lower performances up to
the next reboot (lacking complete accounting).

> Just curious: what does tup stand for?

Tunnel Udp Pointer. Suggestion for better name welcome!

Thanks,

Paolo