netdev - RE: [PATCH V2 net-next] TLP: Don't reschedule PTO when there's one outstanding TLP retransmission

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Date:   Thu, 27 Jul 2017 01:33:51 +0000
From:   maowenan <maowenan@...wei.com>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ncardwell@...gle.com" <ncardwell@...gle.com>,
        "ycheng@...gle.com" <ycheng@...gle.com>,
        "nanditad@...gle.com" <nanditad@...gle.com>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Chenweilong <chenweilong@...wei.com>,
        "Wangkefeng (Kevin)" <wangkefeng.wang@...wei.com>
Subject: RE: [PATCH V2 net-next] TLP: Don't reschedule PTO when there's one
 outstanding TLP retransmission



> -----Original Message-----
> From: Sergei Shtylyov [mailto:sergei.shtylyov@...entembedded.com]
> Sent: Wednesday, July 26, 2017 6:03 PM
> To: maowenan; netdev@...r.kernel.org; davem@...emloft.net;
> ncardwell@...gle.com; ycheng@...gle.com; nanditad@...gle.com;
> weiyongjun (A); Chenweilong; Wangkefeng (Kevin)
> Subject: Re: [PATCH V2 net-next] TLP: Don't reschedule PTO when there's one
> outstanding TLP retransmission
> 
> On 7/26/2017 12:44 PM, Mao Wenan 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 at transmit
> > side, in tcp_ack() will call tcp_schedule_loss_probe() to rearm PTO,
> > the handler tcp_send_loss_probe() is to check
> > if(tp->tlp_high_seq) then go to rearm_timer(because there is one
> > outstanding TLP named A), so the new TLP probe can't be sent out and
> > it needs to rearm the RTO timer(timeout is relative to the transmit
> > time of the write queue head).
> >
> > After that, there is another data packet with ack_seq A is received,
> > if the tlp_time_stamp is greater than rto_time_stamp, it will reset
> > the TLP timeout, which is before previous RTO timeout, so PTO is rearm
> > and previous RTO is cleared. Because there is no retransmission packet
> > was sent or no TLP sack receive, tp->tlp_high_seq can't be reset to
> > zero and the next TLP probe also can't be sent out, so there is no
> > way(or very long time) to retransmit the lost packet.
> >
> > This fix is to check(tp->tlp_high_seq) in tcp_schedule_loss_probe()
> > when TLP PTO is after RTO, It is not needed to reschedule PTO when
> > there is one outstanding TLP retransmission, so if the TLP A is lost
> > RTO can retransmit lost packet, then tp->tlp_high_seq will be set to
> > 0, and TLP will go to the normal work process.
> >
> > v1->v2
> > 	refine some words of code and patch comments.
> >
> > Signed-off-by: Mao Wenan <maowenan@...wei.com>
> > ---
> >   net/ipv4/tcp_output.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index
> > 886d874..f85c7ef 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -2423,6 +2423,12 @@ bool tcp_schedule_loss_probe(struct sock *sk)
> >   	tlp_time_stamp = tcp_jiffies32 + timeout;
> >   	rto_time_stamp = (u32)inet_csk(sk)->icsk_timeout;
> >   	if ((s32)(tlp_time_stamp - rto_time_stamp) > 0) {
> > +		/* It is not needed to reschedule PTO when there
> > +		 * is one outstanding TLP retransmission.
> > +		 */
> > +		if (tp->tlp_high_seq) {
> > +			return false;
> > +		}
> 
>     I have already told you to remove the needless {}... :-/
Thanks, sorry for this.
> 
> [...]
> 
> MBR, Sergei