netdev - Re: [PATCH net] tcp: fix TLP timer not set when CA

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Message-ID: <CADVnQynw7_4wJTUBHTnQ91rEoXKK+LuS1NQHdpYNhQs3CnMfsg@mail.gmail.com>
Date:   Fri, 22 Jan 2021 09:36:57 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Pengcheng Yang <yangpc@...gsu.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: fix TLP timer not set when CA_STATE changes from
 DISORDER to OPEN

On Fri, Jan 22, 2021 at 5:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Jan 22, 2021 at 11:28 AM Pengcheng Yang <yangpc@...gsu.com> wrote:
> >
> > When CA_STATE is in DISORDER, the TLP timer is not set when receiving
> > an ACK (a cumulative ACK covered out-of-order data) causes CA_STATE to
> > change from DISORDER to OPEN. If the sender is app-limited, it can only
> > wait for the RTO timer to expire and retransmit.
> >
> > The reason for this is that the TLP timer is set before CA_STATE changes
> > in tcp_ack(), so we delay the time point of calling tcp_set_xmit_timer()
> > until after tcp_fastretrans_alert() returns and remove the
> > FLAG_SET_XMIT_TIMER from ack_flag when the RACK reorder timer is set.
> >
> > This commit has two additional benefits:
> > 1) Make sure to reset RTO according to RFC6298 when receiving ACK, to
> > avoid spurious RTO caused by RTO timer early expires.
> > 2) Reduce the xmit timer reschedule once per ACK when the RACK reorder
> > timer is set.
> >
> > Link: https://lore.kernel.org/netdev/1611139794-11254-1-git-send-email-yangpc@wangsu.com
> > Signed-off-by: Pengcheng Yang <yangpc@...gsu.com>
> > Cc: Neal Cardwell <ncardwell@...gle.com>
> > ---
>
> This looks like a very nice patch, let me run packetdrill tests on it.
>
> By any chance, have you cooked a packetdrill test showing the issue
> (failing on unpatched kernel) ?

Thanks, Pengcheng. This patch looks good to me as well, assuming it
passes our packetdrill tests. I agree with Eric that it would be good
to have an explicit packetdrill test for this case.

neal