netdev - Re: [PATCH v3] tcp: fix tp->undo_retrans accounting in tcp_sacktag

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Date:   Mon, 13 Sep 2021 09:10:22 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     zhenggy <zhenggy@...natelecom.cn>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, qitiepeng@...natelecom.cn,
        wujianguo@...natelecom.cn, liyonglong@...natelecom.cn
Subject: Re: [PATCH v3] tcp: fix tp->undo_retrans accounting in tcp_sacktag_one()

On Mon, Sep 13, 2021 at 8:49 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, Sep 13, 2021 at 3:51 AM zhenggy <zhenggy@...natelecom.cn> wrote:
> >
> > Commit 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit
> > time") may directly retrans a multiple segments TSO/GSO packet without
> > split, Since this commit, we can no longer assume that a retransmitted
> > packet is a single segment.
> >
> > This patch fixes the tp->undo_retrans accounting in tcp_sacktag_one()
> > that use the actual segments(pcount) of the retransmitted packet.
> >
> > Before that commit (10d3be569243), the assumption underlying the
> > tp->undo_retrans-- seems correct.
> >
> > Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")
> >
>
> nit: We normally do not add an empty line between Fixes: tag and others.
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

This is a nice fix that would increase the effectiveness of TCP undo
in high-speed networks.

>
> > Signed-off-by: zhenggy <zhenggy@...natelecom.cn>
> > ---
> >  net/ipv4/tcp_input.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 3f7bd7a..141e85e 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -1346,7 +1346,7 @@ static u8 tcp_sacktag_one(struct sock *sk,
> >         if (dup_sack && (sacked & TCPCB_RETRANS)) {
> >                 if (tp->undo_marker && tp->undo_retrans > 0 &&
> >                     after(end_seq, tp->undo_marker))
> > -                       tp->undo_retrans--;
> > +                       tp->undo_retrans = max_t(int, 0, tp->undo_retrans - pcount);
> >                 if ((sacked & TCPCB_SACKED_ACKED) &&
> >                     before(start_seq, state->reord))
> >                                 state->reord = start_seq;
> > --
> > 1.8.3.1
> >