netdev - Re: [PATCH net-next 1/4] tcp: stamp SCM_TSTAMP_ACK later in tcp_clean_rtx

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Date:   Sun, 28 Jun 2020 16:57:01 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Yousuk Seung <ysseung@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net-next 1/4] tcp: stamp SCM_TSTAMP_ACK later in tcp_clean_rtx_queue()

On Sat, Jun 27, 2020 at 12:06 AM Yousuk Seung <ysseung@...gle.com> wrote:
>
> Currently tp->delivered is updated with sacked packets but not
> cumulatively acked when SCP_TSTAMP_ACK is timestamped. This patch moves
> a tcp_ack_tstamp() call in tcp_clean_rtx_queue() to later in the loop so
> that when a skb is fully acked OPT_STATS of SCM_TSTAMP_ACK will include
> the current skb in the delivered count. When not fully acked
> tcp_ack_tstamp() is a no-op and there is no change in behavior.
>
> Signed-off-by: Yousuk Seung <ysseung@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Acked-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> ---
>  net/ipv4/tcp_input.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index f3a0eb139b76..2a683e785cca 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3078,8 +3078,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
>                 u8 sacked = scb->sacked;
>                 u32 acked_pcount;
>
> -               tcp_ack_tstamp(sk, skb, prior_snd_una);
> -
>                 /* Determine how many packets and what bytes were acked, tso and else */
>                 if (after(scb->end_seq, tp->snd_una)) {
>                         if (tcp_skb_pcount(skb) == 1 ||
> @@ -3143,6 +3141,8 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
>                 if (!fully_acked)
>                         break;
>
> +               tcp_ack_tstamp(sk, skb, prior_snd_una);
> +
>                 next = skb_rb_next(skb);
>                 if (unlikely(skb == tp->retransmit_skb_hint))
>                         tp->retransmit_skb_hint = NULL;

This moves tcp_ack_tstamp beyond these two breaks:

                tcp_ack_tstamp(sk, skb, prior_snd_una);

                /* Determine how many packets and what bytes were
acked, tso and else */
                if (after(scb->end_seq, tp->snd_una)) {
                        if (tcp_skb_pcount(skb) == 1 ||
                            !after(tp->snd_una, scb->seq))
                                break;

                        acked_pcount = tcp_tso_acked(sk, skb);
                        if (!acked_pcount)
                                break;
                        fully_acked = false;
                } else {
                        acked_pcount = tcp_skb_pcount(skb);
                }

but tcp_ack_tstamp does not necessarily act on the end_seq in the
packet, it acts on shinfo->tskey:

        if (!before(shinfo->tskey, prior_snd_una) &&
            before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
                tcp_skb_tsorted_save(skb) {
                        __skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
                } tcp_skb_tsorted_restore(skb);
        }

on the next call to tcp_clean_rtx_queue tcp_sk(sk)->snd_una will be
beyond tskey, so can this result in missing notifications?