netdev - Re: TCP sender stuck in persist despite peer advertising non-zero window

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Date:   Fri, 16 Oct 2020 09:35:42 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Apollon Oikonomopoulos <apoikos@...sg.gr>,
        Yuchung Cheng <ycheng@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: TCP sender stuck in persist despite peer advertising non-zero window

On Fri, Oct 16, 2020 at 12:37 AM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Thu, Oct 15, 2020 at 6:12 PM Apollon Oikonomopoulos <apoikos@...sg.gr> wrote:
> >
> > Yuchung Cheng <ycheng@...gle.com> writes:
> >
> > > On Thu, Oct 15, 2020 at 1:22 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > >>
> > >> On Thu, Oct 15, 2020 at 2:31 PM Apollon Oikonomopoulos <apoikos@...sg.gr> wrote:
> > >> >
> > >> > Hi,
> > >> >
> > >> > I'm trying to debug a (possible) TCP issue we have been encountering
> > >> > sporadically during the past couple of years. Currently we're running
> > >> > 4.9.144, but we've been observing this since at least 3.16.
> > >> >
> > >> > Tl;DR: I believe we are seeing a case where snd_wl1 fails to be properly
> > >> > updated, leading to inability to recover from a TCP persist state and
> > >> > would appreciate some help debugging this.
> > >>
> > >> Thanks for the detailed report and diagnosis. I think we may need a
> > >> fix something like the following patch below.
> >
> > That was fast, thank you!
> >
> > >>
> > >> Eric/Yuchung/Soheil, what do you think?
> > > wow hard to believe how old this bug can be. The patch looks good but
> > > can Apollon verify this patch fix the issue?
> >
> > Sure, I can give it a try and let the systems do their thing for a couple of
> > days, which should be enough to see if it's fixed.
>
> Great, thanks!
>
> > Neal, would it be possible to re-send the patch as an attachment? The
> > inlined version does not apply cleanly due to linewrapping and
> > whitespace changes and, although I can re-type it, I would prefer to test
> > the exact same thing that would be merged.
>
> Sure, I have attached the "git format-patch" format of the commit. It
> does seem to apply cleanly to the v4.9.144 kernel you mentioned you
> are using.
>
> Thanks for testing this!
>
> best,
> neal

Ouch, this is an interesting bug.  Would netperf -t TCP_RR -- -r
2GB,2GB   " be a possible test ?
(I am afraid packetdrill won't be able to test this in a reasonable
amount of time)

Neal, can you include in your changelog the link to Apollon awesome
email, I think it was a very nice
investigation and Apollon deserves more credit than a mere "Reported-by:" tag ;)

Maybe this one :
Link: https://www.spinics.net/lists/netdev/msg692430.html


Thanks !