netdev - Re: [PATCH net-next] TLP: Don't reschedule PTO when there's one outstanding TLP retransmission.

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Date:   Tue, 25 Jul 2017 09:29:38 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Mao Wenan <maowenan@...wei.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, weiyongjun1@...wei.com,
        Weilong Chen <chenweilong@...wei.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH net-next] TLP: Don't reschedule PTO when there's one
 outstanding TLP retransmission.

On Tue, Jul 25, 2017 at 4:35 AM, Mao Wenan <maowenan@...wei.com> wrote:
> If there is one TLP probe went out(TLP use the write_queue_tail packet
> as TLP probe, we assume this first TLP probe named A), and this TLP
> probe was not acked by receive side.
>
> Then the transmit side sent the next two packetes out(named B,C), but
> unfortunately these two packets are also not acked by receive side.
>
> And then there is one data packet with ack_seq A arrive, in tcp_ack()
> will call tcp_schedule_loss_probe() to rearm PTO, the handler
> tcp_send_loss_probe() pass if(tp->tlp_high_seq)(because there is
> one outstanding TLP named A,tp->tlp_high_seq is not zero),
> so the new TLP probe can't be went out and need to rearm the RTO
> timer(timeout is relative to the transmit time of the write queue head).
>
> After this, another data packet with ack_seq A is received,
> if the tlp_time_stamp is after rto_time_stamp, it will reset the
> TLP timeout with delta value, which is before previous RTO timeout,
> so PTO is rearm and previous RTO is cleared. This TLP probe also can't
> be sent out because of tp->tlp_high_seq != 0, so there is no way(or need
> very long time)to retransmit the packet because of TLP A is lost.
>
> This fix is not to pass the if(tp->tlp_high_seq) in tcp_schedule_loss_probe()
> when TLP PTO is after RTO, It is no need to reschedule PTO when there
> is one outstanding TLP retransmission, so if the TLP A is lost then RTO can
> retransmit that packet, and tp->tlp_high_seq will be set to 0. After this TLP
> will go to the normal work process.
>
> Signed-off-by: Mao Wenan <maowenan@...wei.com>

Thanks for posting this. This is a pretty involved scenario. To help
document/test precisely what the behavior is before and after your
patch, would you be able to post a packetdrill (
https://github.com/google/packetdrill ) test case for this scenario?

Can I ask if you saw this scenario in an actual trace, or noticed this
by inspection?

thanks,
neal