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Message-ID: <6ff1ef1f-1c3a-8822-eb82-a4dde482de93@chinatelecom.cn>
Date: Mon, 11 Jul 2022 09:53:23 +0800
From: Yonglong Li <liyonglong@...natelecom.cn>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev <netdev@...r.kernel.org>,
David Miller <davem@...emloft.net>,
David Ahern <dsahern@...nel.org>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v2] tcp: make retransmitted SKB fit into the send window
Hi Eric,
thank you for your feedback and suggestions. Sorry for miss part of your
prior feedback. I will prepare a v3 patch as your suggestion.
And anther your feedback about "need a Fixes: tag", I can not find any commit
which is related to this patch. Do you have any suggestion about Fixes:tag?
On 7/8/2022 7:45 PM, Eric Dumazet wrote:
> On Fri, Jul 8, 2022 at 10:59 AM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>>
>> From: lyl <liyonglong@...natelecom.cn>
>>
>> current code of __tcp_retransmit_skb only check TCP_SKB_CB(skb)->seq
>> in send window, and TCP_SKB_CB(skb)->seq_end maybe out of send window.
>> If receiver has shrunk his window, and skb is out of new window, it
>> should not retransmit it.
>
> More exactly, it should retransmit a smaller portion of the payload.
>
>>
>> test packetdrill script:
>> 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
>> +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
>> +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
>>
>> +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
>> +0 > S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8>
>> +.05 < S. 0:0(0) ack 1 win 6000 <mss 1000,nop,nop,sackOK>
>> +0 > . 1:1(0) ack 1
>>
>> +0 write(3, ..., 10000) = 10000
>>
>> +0 > . 1:2001(2000) ack 1 win 65535
>> +0 > . 2001:4001(2000) ack 1 win 65535
>> +0 > . 4001:6001(2000) ack 1 win 65535
>>
>> +.05 < . 1:1(0) ack 4001 win 1001
>>
>> and tcpdump show:
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 1:2001, ack 1, win 65535, length 2000
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 2001:4001, ack 1, win 65535, length 2000
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
>> 192.0.2.1.8080 > 192.168.226.67.55: Flags [.], ack 4001, win 1001, length 0
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
>> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
>>
>> when cient retract window to 1001, send window is [4001,5002],
>> but TLP send 5001-6001 packet which is out of send window.
>>
>> Signed-off-by: liyonglong <liyonglong@...natelecom.cn>
>> ---
>> net/ipv4/tcp_output.c | 7 ++++++-
>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> index 1c05443..7e1569e 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -3176,7 +3176,12 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
>> TCP_SKB_CB(skb)->seq != tp->snd_una)
>> return -EAGAIN;
>>
>> - len = cur_mss * segs;
>> + len = min_t(int, tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq, cur_mss * segs);
>> +
>> + /* retransmit serves as a zero window probe. */
>> + if (len == 0 && TCP_SKB_CB(skb)->seq == tp->snd_una)
>> + len = cur_mss;
>> +
>
> I asked in my prior feeback to align len to a multiple of cur_mss.
> Trying to send full MSS is likely helping receivers to not waste
> memory and thus send us bigger RWIN.
>
> Also I think the logic about zero window probe could be consolidated
> in one place.
>
> Also you forgot to change the part about tcp_retrans_try_collapse()
> which I also mentioned in my feedback.
>
> If we want to fix this problem for good, we need to take care of all cases...
>
> Something like this:
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 18c913a2347a984ae8cf2793bb8991e59e5e94ab..2aa67cbfa1428a11b723263083ce48284762e306
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3144,7 +3144,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct
> sk_buff *skb, int segs)
> struct tcp_sock *tp = tcp_sk(sk);
> unsigned int cur_mss;
> int diff, len, err;
> -
> + int avail_wnd;
>
> /* Inconclusive MTU probe */
> if (icsk->icsk_mtup.probe_size)
> @@ -3167,16 +3167,24 @@ int __tcp_retransmit_skb(struct sock *sk,
> struct sk_buff *skb, int segs)
>
> cur_mss = tcp_current_mss(sk);
>
> + avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
> /* If receiver has shrunk his window, and skb is out of
> * new window, do not retransmit it. The exception is the
> * case, when window is shrunk to zero. In this case
> - * our retransmit serves as a zero window probe.
> + * our retransmit of one segment serves as a zero window probe.
> */
> - if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
> - TCP_SKB_CB(skb)->seq != tp->snd_una)
> - return -EAGAIN;
> + if (avail_wnd <= 0) {
> + if (TCP_SKB_CB(skb)->seq != tp->snd_una)
> + return -EAGAIN;
> + avail_wnd = cur_mss;
> + }
>
> len = cur_mss * segs;
> + if (len > avail_wnd) {
> + len = rounddown(avail_wnd, cur_mss);
> + if (!len)
> + len = avail_wnd;
> + }
> if (skb->len > len) {
> if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
> cur_mss, GFP_ATOMIC))
> @@ -3190,8 +3198,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct
> sk_buff *skb, int segs)
> diff -= tcp_skb_pcount(skb);
> if (diff)
> tcp_adjust_pcount(sk, skb, diff);
> - if (skb->len < cur_mss)
> - tcp_retrans_try_collapse(sk, skb, cur_mss);
> + avail_wnd = min_t(int, avail_wnd, cur_mss);
> + if (skb->len < avail_wnd)
> + tcp_retrans_try_collapse(sk, skb, avail_wnd);
> }
>
> /* RFC3168, section 6.1.1.1. ECN fallback */
>
--
Li YongLong
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