Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode

From: Eric Dumazet <eric.dumazet@gmail.com>
To: Craig Gallek <kraigatgoog@gmail.com>
Cc: Linux Kernel Network Developers <netdev@vger.kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode
Date: Fri, 25 Mar 2016 10:00:42 -0700	[thread overview]
Message-ID: <1458925242.6473.41.camel@edumazet-glaptop3.roam.corp.google.com> (raw)
In-Reply-To: <CAEfhGiy-V0da6LQ_SY+E4u7S49n5dEpVaKiOOo103ijQtPujAw@mail.gmail.com>

On Fri, 2016-03-25 at 12:31 -0400, Craig Gallek wrote:

> I believe the issue here is that closing the listen sockets will drop
> any connections that are in the listen queue but have not been
> accepted yet.  In the case of reuseport, you could in theory drain
> those queues into the non-closed sockets, but that probably has some
> interesting consequences...

It is more complicated than this.

Ideally, no TCP connection should be dropped during a server change.

The idea is to let old program running as long as :
1) It has established TCP sessions
2) Some SYN_RECV pseudo requests are still around

Once 3WHS completes for these SYN_RECV, children are queued into
listener accept queues.

But the idea is to direct all new SYN packets to the 'new' process and
its listeners. (New SYN_RECV should be created on behalf on the new
listeners only)

In some environments, the listeners are simply transfered via FD
passing, from the 'old process' to the new one.