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

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Date:   Wed, 26 Jul 2017 02:12:29 +0000
From:   maowenan <maowenan@...wei.com>
To:     Neal Cardwell <ncardwell@...gle.com>
CC:     Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Chenweilong <chenweilong@...wei.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        "Nandita Dukkipati" <nanditad@...gle.com>,
        "Wangkefeng (Kevin)" <wangkefeng.wang@...wei.com>
Subject: RE: [PATCH net-next] TLP: Don't reschedule PTO when there's one
 outstanding TLP retransmission.



> -----Original Message-----
> From: Neal Cardwell [mailto:ncardwell@...gle.com]
> Sent: Tuesday, July 25, 2017 9:30 PM
> To: maowenan
> Cc: Netdev; David Miller; weiyongjun (A); Chenweilong; Yuchung Cheng;
> Nandita Dukkipati
> 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?
[Mao Wenan] It is my actual product scenario, from some debug trace info 
I found that PTO is always rescheduled before RTO, this is PTO is trigged 
by receiving one tcp_ack packets. After this patch, the product works well and 
there is no previous issue happen. The packet can retransmit with RTO if TLP probe 
is lost. 
I will say sorry that my local test can't reproduce because i can't build successfully the same 
complex scenario, I don't have mature test case to reproduce now but I will do my best to try
in local test environment. 
Thank you.
> 
> thanks,
> neal