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

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Message-ID: <9ef3bfe-01f-29da-6d5-1baf2fad7254@ewheeler.net>
Date: Sun, 15 Jun 2025 13:00:44 -0700 (PDT)
From: Eric Wheeler <netdev@...ts.ewheeler.net>
To: Neal Cardwell <ncardwell@...gle.com>
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 Tue, 10 Jun 2025, Neal Cardwell 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?

Sorry for the radio silence, I just got back in town so I can do that 
later this week.  

> 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.

I can tested on top of 6.6-stable but I have to put a production system 
into standby in order to do that, so I will report back when I can, 
possibly as late as Friday 6/20 because the office is closed that day and 
I can work on it.
 
> If you have cycles to rerun your iperf test, with  tcpdump, nstat, and
> ss instrumentation, that would be fantastic!

will do 
 
> The patch passes our internal packetdrill test suite, including new
> tests for this issue (based on the packetdrill scripts posted earlier
> in this thread.

Awesome thank you for all of the effort to fix this!

-Eric

> 
> But it would be fantastic to directly confirm that this fixes your issue.
> 
> Thanks!
> neal
>