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

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Date:	Wed, 16 Dec 2015 17:15:14 +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, Dec 16, 2015 at 08:38:14AM +0100, Willy Tarreau wrote:
> On Tue, Dec 15, 2015 at 01:21:15PM -0800, Eric Dumazet wrote:
> > On Tue, 2015-12-15 at 20:44 +0100, Willy Tarreau wrote:
> > 
> > > Thus do you think it's worth adding a new option as Tolga proposed ?
> > 
> > 
> > I thought we tried hard to avoid adding the option but determined
> > we could not avoid it ;)
> 
> Not yet, your other proposal of disabling SO_REUSEPORT makes sense if
> we combine it with the proposal to change the score in my patch. If
> we say that a socket which has SO_REUSEPORT scores higher, then the
> connections which don't want to accept new connections anymore will
> simply have to drop it an not be elected. I find this even cleaner
> since the sole purpose of the loop is to find the best socket in case
> of SO_REUSEPORT.

So I tried this and am pretty satisfied with the results, as I couldn't
see any single reset on 4.4-rc5 with it. On 4.1 I got a few very rare
resets at the exact moment the new process binds to the socket, because
I suspect some ACKs end up in the wrong queue exactly there. But
apparently the changes you did in 4.4 totally got rid of this, which is
great!

I suspected that I could enter a situation where a new process could
fail to bind if generations n-1 and n-2 were still present, because
n-2 would be running without SO_REUSEPORT and that should make this
test fail in inet_csk_bind_conflict(), but it never failed for me :

                        if ((!reuse || !sk2->sk_reuse ||
                            sk2->sk_state == TCP_LISTEN) &&
                            (!reuseport || !sk2->sk_reuseport ||
                            (sk2->sk_state != TCP_TIME_WAIT &&
                             !uid_eq(uid, sock_i_uid(sk2))))) {
...

So I'm clearly missing something and can't spot what. I mean, I'd
prefer to see my patch occasionally fail than not understanding why
it always works! If anyone has an suggestion I'm interested.

Here's the updated patch.

Best regards,
Willy


View attachment "0001-net-make-lingering-sockets-score-less-in-compute_sco.patch" of type "text/plain" (2648 bytes)