Subject: Re: [PATCH v2 net-next] net: Add low-latency/polling support for UDP multicast

From: Eliezer Tamir <[email-masked]>
Date: Thu, 08 Aug 2013 11:46:17 +0300

On 07/08/2013 23:22, Eric Dumazet wrote:
> On Tue, 2013-08-06 at 14:51 -0500, Shawn Bohrer wrote:
>> Set the napi id for each socket in the multicast path to enable
>> low-latency/polling support.
>>
>> Signed-off-by: Shawn Bohrer <[email-masked]>
>> ---
>> v2 include ipv6 support
> 
> This might help your workload, but I doubt it is generic enough.
> 
> One UDP socket is supposed to receive traffic from many endpoints,
> so we have no guarantee all the received traffic will end on a single RX
> queue on the NIC.
> 
> That's the same logic than RFS here.
> 
> sk_mark_napi_id() in UDP are wrong IMHO.
> 
> It should be guarded by the following test in  
> __udp_queue_rcv_skb()
> 
> if (inet_sk(sk)->inet_daddr) {
>     sock_rps_save_rxhash(sk, skb);
>     sk_mark_napi_id(sk, skb);
> }
> 
> (To occur only for connected UDP sockets, where we are 100% sure all
> packets will use this same rxhash/rx queue)

This would also be safe if there is only one NIC and said NIC was
programmed to always place this socket's data on the same queue.

I don't have a good suggestion on how to detect this.