netdev - Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is received

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Date:	Fri, 9 Jan 2015 11:43:54 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	Sébastien Barré <sebastien.barre@...ouvain.be>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Gregory Detal <gregory.detal@...ouvain.be>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is received

On Thu, Jan 8, 2015 at 8:25 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> On Thu, 2015-01-08 at 10:49 -0500, Neal Cardwell wrote:
> > On Thu, Jan 8, 2015 at 10:39 AM, Sébastien Barré
> > <sebastien.barre@...ouvain.be> wrote:
> > > What do you and Neal think ?
> >
> > My preference is to have the whole expression detecting the case where
> > the receiver got the probe packet encoded in a single expression. I
> > don't have a strong feeling about whether it should be stored in a
> > bool (to reduce the size of the diff) or written directly into the if
> > () expression (to reduce the size of the code). I'll defer to Eric on
> > which he thinks is better. :-)
>
> There is no shame using helpers with nice names to help understand this
> TCP stack. Even if the helper is used exactly once.
>
> In this case, it seems we test 2 different conditions, so this could use
> 2 helpers with self describing names.
>
> When I see :
>
> > +     if (((ack == tp->tlp_high_seq) &&
> > +          !(flag & (FLAG_SND_UNA_ADVANCED |
> > +                    FLAG_NOT_DUP | FLAG_DATA_SACKED))) ||
> > +         (!before(ack, tp->tlp_high_seq) && (flag & FLAG_DSACKING_ACK))) {
> >               tp->tlp_high_seq = 0;
>
> My brain hurts.
Sebastien: I suggest breaking down by ACK types for readability. e.g.,

/* This routine deals with acks during a TLP episode.
 * We mark the end of a TLP episode on receiving TLP dupack or when
 * ack is after tlp_high_seq.
 * Ref: loss detection algorithm in draft-dukkipati-tcpm-tcp-loss-probe.
 */
static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (before(ack, tp->tlp_high_seq))
                return;

        if (flag & FLAG_DSACKING_ACK) {
                /* This DSACK means original and TLP probe arrived; no loss */
                tp->tlp_high_seq = 0;
        } else if (after(ack, tp->tlp_high_seq)) {
                /* ACK advances: there was a loss, so reduce cwnd. Reset
                 * tlp_high_seq in tcp_init_cwnd_reduction()
                 */
                tcp_init_cwnd_reduction(sk);
                tcp_set_ca_state(sk, TCP_CA_CWR);
                tcp_end_cwnd_reduction(sk);
                tcp_try_keep_open(sk);
                NET_INC_STATS_BH(sock_net(sk),
                                 LINUX_MIB_TCPLOSSPROBERECOVERY);
        } else if (!(flag & (FLAG_SND_UNA_ADVANCED |
                             FLAG_NOT_DUP | FLAG_DATA_SACKED))) {
                /* Pure dupack: original and TLP probe arrived; no loss */
                tp->tlp_high_seq = 0;
        }
}

>
>
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