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

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Message-ID: <1416003883.17262.72.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 14 Nov 2014 14:24:43 -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 14:10 -0800, Andy Lutomirski wrote:

> I have a bunch of threads that are pinned to various CPUs or groups of
> CPUs.  Each thread is responsible for a fixed set of flows.  I'd like
> those flows to go to those CPUs.
> 
> RFS will eventually do it, but it would be nice if I could
> deterministically ask for a flow to be routed to the right CPU.  Also,
> if my thread bounces temporarily to another CPU, I don't really need
> the flow to follow it -- I'd like it to stay put.
> 
> This has a significant benefit over using automatic steering: with
> automatic steering, I have to make all of the hash tables have a size
> around the square of the total number of the flows in order to make it
> reliable.
> 
> Something like SO_STEER_TO_THIS_CPU would be fine, as long as it
> reported whether it worked (for my diagnostics).

This requires some kind of hardware support, and unfortunately this is
not generic.

With SO_INCOMING_CPU, you simply can pass fd of sockets around threads,
so that a dumb RSS multiqueue NIC is OK (assuming you are not using some
encapsulation that NIC is not able to parse to find L4 information)

Steering is a dream, I really think its easier to build flows so that
their RX queue matches your requirements.

We usually can pick at least one element of the 4-tuple, so its actually
possible to get this before connect().


Two cases :

1) Passive connections.

   After accept(), get SO_INCOMING_CPU, then pass the fd to appropriate
thread of your pool.

2) Active connections .
   find a proper 4-tuple, bind() then connect(). Eventually check
SO_INCOMING_CPU to verify your expectations.



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