netdev - Re: [PATCH net-next] net: introduce SO_INCOMING

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Date:	Fri, 14 Nov 2014 14:18:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Ying Cai <ycai@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH net-next] net: introduce SO_INCOMING_CPU

On Fri, Nov 14, 2014 at 2:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2014-11-14 at 12:16 -0800, Andy Lutomirski wrote:
>
>> Sure, but how do I know that it'll be the same CPU next time?
>
> Because the NIC always use same RX queue for a given flow.
>
> So if you setup your IRQ affinities properly, the same CPU will drain
> packets from this RX queue. And since RFS is off, you have the guarantee
> the same CPU will be used to process packets in TCP stack.

Right.  My concern is that if RFS is off, everything works fine, but
if RFS is on (and the app has no good way to know), then silly results
will happen.  I think I'd rather have the getsockopt fail if RFS is on
or at least give some indication so that the app can react
accordingly.

--Andy

>
> This SO_INCOMING_CPU info is a hint, there is no guarantee eg if you use
> bonding and some load balancer or switch decides to send packets on
> different links.
>
> Most NIC use Toeplitz hash, so given the 4-tuple, and rss key (40
> bytes), you can actually compute the hash in software and know on which
> RX queue traffic should land.
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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