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

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Message-ID: <1416003371.17262.66.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 14 Nov 2014 14:16:11 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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, 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.

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.

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