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

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Date:	Sun, 11 Aug 2013 10:59:39 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Shawn Bohrer <sbohrer@...advisors.com>, davem@...emloft.net,
	netdev@...r.kernel.org, Amir Vadai <amirv@...lanox.com>,
	tomk@...advisors.com
Subject: Re: [PATCH v2 net-next] net: Add low-latency/polling support for
 UDP multicast

On 09/08/2013 02:55, Eric Dumazet wrote:
> On Thu, 2013-08-08 at 11:46 +0300, Eliezer Tamir wrote:
>> On 07/08/2013 23:22, Eric Dumazet wrote:
>>> 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.
> 
> Well, this stuff relies on flows being correctly steered.
> 
> TCP stack performs much better if we avoid reorders ;)

Let's limit the discussion to UDP for now.
If you are getting packets on multiple queues for a TCP socket,
it's hard to see how you can avoid caused reordering.

Maybe it would be enough to have a socket option that the user
can set to tell the stack "I know what I'm doing, please allow busy
polling on this UDP socket, even though it's not bound".
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