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Date: Tue, 14 Jan 2020 10:39:40 -0800 From: Yuchung Cheng <ycheng@...gle.com> To: Eric Dumazet <edumazet@...gle.com> Cc: Pengcheng Yang <yangpc@...gsu.com>, David Miller <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, andriin@...com, netdev <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] tcp: fix marked lost packets not being retransmitted On Tue, Jan 14, 2020 at 8:06 AM Eric Dumazet <edumazet@...gle.com> wrote: > > On Tue, Jan 14, 2020 at 1:24 AM Pengcheng Yang <yangpc@...gsu.com> wrote: > > > > When the packet pointed to by retransmit_skb_hint is unlinked by ACK, > > retransmit_skb_hint will be set to NULL in tcp_clean_rtx_queue(). > > If packet loss is detected at this time, retransmit_skb_hint will be set > > to point to the current packet loss in tcp_verify_retransmit_hint(), > > then the packets that were previously marked lost but not retransmitted > > due to the restriction of cwnd will be skipped and cannot be > > retransmitted. > > > "cannot be retransmittted" sounds quite alarming. > > You meant they will eventually be retransmitted, or that the flow is > completely frozen at this point ? He probably means those lost packets will be skipped until a timeout that reset hint pointer. nice fix this would save some RTOs. > > Thanks for the fix and test ! > > (Not sure why you CC all these people having little TCP expertise btw) > > > To fix this, when retransmit_skb_hint is NULL, retransmit_skb_hint can > > be reset only after all marked lost packets are retransmitted > > (retrans_out >= lost_out), otherwise we need to traverse from > > tcp_rtx_queue_head in tcp_xmit_retransmit_queue(). > > > > Packetdrill to demonstrate: > > > > // Disable RACK and set max_reordering to keep things simple > > 0 `sysctl -q net.ipv4.tcp_recovery=0` > > +0 `sysctl -q net.ipv4.tcp_max_reordering=3` > > > > // Establish a connection > > +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 > > +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 > > +0 bind(3, ..., ...) = 0 > > +0 listen(3, 1) = 0 > > > > +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7> > > +0 > S. 0:0(0) ack 1 <...> > > +.01 < . 1:1(0) ack 1 win 257 > > +0 accept(3, ..., ...) = 4 > > > > // Send 8 data segments > > +0 write(4, ..., 8000) = 8000 > > +0 > P. 1:8001(8000) ack 1 > > > > // Enter recovery and 1:3001 is marked lost > > +.01 < . 1:1(0) ack 1 win 257 <sack 3001:4001,nop,nop> > > +0 < . 1:1(0) ack 1 win 257 <sack 5001:6001 3001:4001,nop,nop> > > +0 < . 1:1(0) ack 1 win 257 <sack 5001:7001 3001:4001,nop,nop> > > > > // Retransmit 1:1001, now retransmit_skb_hint points to 1001:2001 > > +0 > . 1:1001(1000) ack 1 > > > > // 1001:2001 was ACKed causing retransmit_skb_hint to be set to NULL > > +.01 < . 1:1(0) ack 2001 win 257 <sack 5001:8001 3001:4001,nop,nop> > > // Now retransmit_skb_hint points to 4001:5001 which is now marked lost > > > > // BUG: 2001:3001 was not retransmitted > > +0 > . 2001:3001(1000) ack 1 > > > > Signed-off-by: Pengcheng Yang <yangpc@...gsu.com> > > --- > > net/ipv4/tcp_input.c | 7 ++++--- > > 1 file changed, 4 insertions(+), 3 deletions(-) > > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > > index 0238b55..5347ab2 100644 > > --- a/net/ipv4/tcp_input.c > > +++ b/net/ipv4/tcp_input.c > > @@ -915,9 +915,10 @@ static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq, > > /* This must be called before lost_out is incremented */ > > static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb) > > { > > - if (!tp->retransmit_skb_hint || > > - before(TCP_SKB_CB(skb)->seq, > > - TCP_SKB_CB(tp->retransmit_skb_hint)->seq)) > > + if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) || > > + (tp->retransmit_skb_hint && > > + before(TCP_SKB_CB(skb)->seq, > > + TCP_SKB_CB(tp->retransmit_skb_hint)->seq))) > > tp->retransmit_skb_hint = skb; > > } > > > > -- > > 1.8.3.1 > >
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