netdev - Re: [patch] socket.7: Document SO_INCOMING

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Date:   Sun, 19 Feb 2017 21:55:20 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Francois Saint-Jacques <fsaintjacques@...il.com>
Cc:     mtk.manpages@...il.com, linux-man@...r.kernel.org,
        netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch] socket.7: Document SO_INCOMING_CPU

[CC += Eric, so that he might review]

Hello Francois,

On 02/18/2017 05:06 AM, Francois Saint-Jacques wrote:
> This socket option is undocumented. Applies on the latest version
> (man-pages-4.09-511).
> 
> diff --git a/man7/socket.7 b/man7/socket.7
> index 3efd7a5d8..1a3ffa253 100644
> --- a/man7/socket.7
> +++ b/man7/socket.7
> @@ -490,6 +490,26 @@ flag on a socket
>  operation.
>  Expects an integer boolean flag.
>  .TP
> +.BR SO_INCOMING_CPU " (getsockopt since Linux 3.19, setsockopt since
> Linux 4.4)"
> +.\" getsocktop 2c8c56e15df3d4c2af3d656e44feb18789f75837
> +.\" setsocktop 70da268b569d32a9fddeea85dc18043de9d89f89
> +Sets or gets the cpu affinity of a socket. Expects an integer flag.
> +.sp
> +.in +4n
> +.nf
> +int cpu = 1;
> +socklen_t len = sizeof(cpu);
> +setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, &len);
> +.fi
> +.in
> +.sp
> +The typical use case is one listener per RX queue, as the associated listener
> +should only accept flows handled in softirq by the same cpu.  This provides
> +optimal NUMA behavior and keep cpu caches hot.
> +.TP
>  .B SO_KEEPALIVE
>  Enable sending of keep-alive messages on connection-oriented sockets.
>  Expects an integer boolean flag.

Thank you! Patch applied.

I have tried to enhance the description somewhat. I'm not sure whether
what I've written is quite correct (or whether it should be further
extended). Eric, could you please take a look at the following, and let 
me know if anything needs fixing:

       SO_INCOMING_CPU  (gettable  since Linux 3.19, settable since Linux
       4.4)
              Sets or gets the CPU affinity  of  a  socket.   Expects  an
              integer flag.

                  int cpu = 1;
                  socklen_t len = sizeof(cpu);
                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, &len);

              Because  all  of the packets for a single stream (i.e., all
              packets for the same 4-tuple) arrive on the single RX queue
              that  is  associated with a particular CPU, the typical use
              case is to employ one listening process per RX queue,  with
              the  incoming  flow being handled by a listener on the same
              CPU that is handling the RX queue.  This  provides  optimal
              NUMA behavior and keeps CPU caches hot.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/