netdev - Re: [BISECT] regression: tcp: fix to allow timestamp undo if no retransmits were sent

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Message-ID: <CADVnQymyjoCnO5S8C33X6=2WjZ-2Rcbrz1LXJ+kkP4OwFSS=jQ@mail.gmail.com>
Date: Fri, 13 Jun 2025 17:02:21 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Eric Wheeler <netdev@...ts.ewheeler.net>
Cc: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, 
	Geumhwan Yu <geumhwan.yu@...sung.com>, Jakub Kicinski <kuba@...nel.org>, 
	Sasha Levin <sashal@...nel.org>, Yuchung Cheng <ycheng@...gle.com>, stable@...nel.org
Subject: Re: [BISECT] regression: tcp: fix to allow timestamp undo if no
 retransmits were sent

On Thu, Jun 12, 2025 at 2:23 PM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Tue, Jun 10, 2025 at 1:15 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > On Mon, Jun 9, 2025 at 1:45 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > >
> > > On Sat, Jun 7, 2025 at 7:26 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > > >
> > > > On Sat, Jun 7, 2025 at 6:54 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > > > >
> > > > > On Sat, Jun 7, 2025 at 3:13 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > > > > >
> > > > > > On Fri, Jun 6, 2025 at 6:34 PM Eric Wheeler <netdev@...ts.ewheeler.net> wrote:
> > > > > > >
> > > > > > > On Fri, 6 Jun 2025, Neal Cardwell wrote:
> > > > > > > > On Thu, Jun 5, 2025 at 9:33 PM Eric Wheeler <netdev@...ts.ewheeler.net> wrote:
> > > > > > > > >
> > > > > > > > > Hello Neal,
> > > > > > > > >
> > > > > > > > > After upgrading to Linux v6.6.85 on an older Supermicro SYS-2026T-6RFT+
> > > > > > > > > with an Intel 82599ES 10GbE NIC (ixgbe) linked to a Netgear GS728TXS at
> > > > > > > > > 10GbE via one SFP+ DAC (no bonding), we found TCP performance with
> > > > > > > > > existing devices on 1Gbit ports was <60Mbit; however, TCP with devices
> > > > > > > > > across the switch on 10Gbit ports runs at full 10GbE.
> > > > > > > > >
> > > > > > > > > Interestingly, the problem only presents itself when transmitting
> > > > > > > > > from Linux; receive traffic (to Linux) performs just fine:
> > > > > > > > >         ~60Mbit: Linux v6.6.85 =TX=> 10GbE -> switch -> 1GbE  -> device
> > > > > > > > >          ~1Gbit: device        =TX=>  1GbE -> switch -> 10GbE -> Linux v6.6.85
> > > > > > > > >
> > > > > > > > > Through bisection, we found this first-bad commit:
> > > > > > > > >
> > > > > > > > >         tcp: fix to allow timestamp undo if no retransmits were sent
> > > > > > > > >                 upstream:       e37ab7373696e650d3b6262a5b882aadad69bb9e
> > > > > > > > >                 stable 6.6.y:   e676ca60ad2a6fdeb718b5e7a337a8fb1591d45f
> >
> > Hi Eric,
> >
> > Do you have cycles to test a proposed fix patch developed by our team?
> >
> > The attached patch should apply (with "git am") for any recent kernel
> > that has the "tcp: fix to allow timestamp undo if no retransmits were
> > sent" patch it is fixing. So you should be able to test it on top of
> > the 6.6 stable or 6.15 stable kernels you used earlier. Whichever is
> > easier.
> >
> > If you have cycles to rerun your iperf test, with  tcpdump, nstat, and
> > ss instrumentation, that would be fantastic!
> >
> > The patch passes our internal packetdrill test suite, including new
> > tests for this issue (based on the packetdrill scripts posted earlier
> > in this thread.
> >
> > But it would be fantastic to directly confirm that this fixes your issue.
>
> Hi Eric (Wheeler),
>
> Just checking: would you be able to test that patch (from my previous
> message) in your environment?
>
> If not, given that the patch fixes our packetdrill reproducers, we can
> send the patch to the list as-is without that testing.

Update on this thread; I sent the patch to the netdev list:

[PATCH net] tcp: fix tcp_packet_delayed() for
tcp_is_non_sack_preventing_reopen() behavior

https://lore.kernel.org/netdev/20250613193056.1585351-1-ncardwell.sw@gmail.com/

best,
neal