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

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Date:   Thu, 27 Jul 2017 01:28:34 +0000
From:   maowenan <maowenan@...wei.com>
To:     Yuchung Cheng <ycheng@...gle.com>
CC:     Neal Cardwell <ncardwell@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Chenweilong <chenweilong@...wei.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: Yuchung Cheng [mailto:ycheng@...gle.com]
> Sent: Thursday, July 27, 2017 12:45 AM
> To: maowenan
> Cc: Neal Cardwell; Netdev; David Miller; weiyongjun (A); Chenweilong; Nandita
> Dukkipati; Wangkefeng (Kevin)
> Subject: Re: [PATCH net-next] TLP: Don't reschedule PTO when there's one
> outstanding TLP retransmission.
> 
> On Tue, Jul 25, 2017 at 7:12 PM, maowenan <maowenan@...wei.com>
> wrote:
> >
> >
> >
> > > -----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
> > > > tp->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.
> tcpdump output of an example trace also helps. Feel free to remove IP address,
> ports, or payload information. We only need the timing and TCP header info.
Ok, please check as below. Server: 192.169.0.18, client: 192.169.0.17
......
20:36:12.239562 IP 192.169.0.18.1234 > 192.169.0.17.56246: P 501216544:501216616(72) ack 2308880351 win 92
20:36:12.240034 IP 192.169.0.18.1234 > 192.169.0.17.56246: P 501216616:501216688(72) ack 2308880351 win 92
20:36:12.260764 IP 192.169.0.18.1234 > 192.169.0.17.56246: P 501216616:501216688(72) ack 2308880351 win 92     /* first TLP probe A, seq: 501216616 */ 
20:36:13.018020 IP 192.169.0.17.56246 > 192.169.0.18.1234: . ack 501216616 win 1970                         
20:36:13.018386 IP 192.169.0.18.1234 > 192.169.0.17.56246: P 501216688:501216760(72) ack 2308880351 win 92     /* server send packet B */
20:36:13.022448 IP 192.169.0.18.1234 > 192.169.0.17.56246: P 501216760:501216832(72) ack 2308880351 win 92     /* server send packet C */
20:36:13.033813 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880351:2308880437(86) ack 501216616 win 1970  /* receive client data packet ack 501216616 */
20:36:13.034213 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880437:2308880523(86) ack 501216616 win 1970  /* receive client data packet ack 501216616 */
20:36:13.034633 IP 192.169.0.18.1234 > 192.169.0.17.56246: . ack 2308880523 win 92                            /* server pure ack client */
20:36:13.039512 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880523:2308880659(136) ack 501216616 win 1970
20:36:13.040083 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880659:2308880795(136) ack 501216616 win 1970
20:36:13.040491 IP 192.169.0.18.1234 > 192.169.0.17.56246: . ack 2308880795 win 92
20:36:13.041161 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880795:2308880931(136) ack 501216616 win 1970
20:36:13.042828 IP 192.169.0.17.56246 > 192.169.0.18.1234: P 2308880931:2308881032(101) ack 501216616 win 1970
20:36:13.043198 IP 192.169.0.18.1234 > 192.169.0.17.56246: . ack 2308881032 win 92
......
> 
> 
> > Thank you.
> > >
> > > thanks,
> > > neal