netdev - Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN

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Date:	Tue, 15 Dec 2015 17:14:56 +0100
From:	Willy Tarreau <w@....eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	Tolga Ceylan <tolga.ceylan@...il.com>,
	Aaron Conole <aconole@...heb.org>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode

Hi Eric,

On Wed, Nov 11, 2015 at 05:09:01PM -0800, Eric Dumazet wrote:
> On Wed, 2015-11-11 at 10:43 -0800, Eric Dumazet wrote:
> > On Wed, 2015-11-11 at 10:23 -0800, Tom Herbert wrote:
> > 
> > > How about doing this in shutdown called for a listener?
> > 
> > Seems a good idea, I will try it, thanks !
> > 
> 
> Arg, I forgot about this shutdown() discussion we had recently
> with Oracle guys.
> 
> It is currently used in linux to unblock potential threads in accept()
> system call.
> 
> This would prevent syn_recv sockets to be finally accepted.

I had a conversation with an haproxy user who's concerned with the
connection drops during the reload operation and we stumbled upon
this thread. I was considering improving shutdown() as well for this
as haproxy already performs a shutdown(RD) during a "pause" operation
(ie: workaround for kernels missing SO_REUSEPORT).

And I found that the code clearly doesn't make this possible since
shutdown(RD) flushes the queue and stops the listening.

However I found what I consider an elegant solution which works
pretty well : by simply adding a test in compute_score(), we can
ensure that a previous socket ranks lower than the current ones,
and is never considered as long as the new ones are present. Here I
achieved this using setsockopt(SO_LINGER). The old process just has
to do this with a non-zero value on the socket it wants to put into
lingering mode and that's all.

I find this elegant since it keeps the same semantics as for a
connected socket in that it avoids killing the queue, and that it
doesn't change the behaviour for existing applications. It just
turns out that listening sockets are set up without any lingering
by default so we don't need to add any new socket options nor
anything.

Please let me know what you think about it (patch attached), if
it's accepted it's trivial to adapt haproxy to this new behaviour.

Thanks!
Willy

View attachment "0001-net-make-lingering-sockets-score-less-in-compute_sco.patch" of type "text/plain" (2495 bytes)