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Date:	Tue, 21 May 2013 18:27:09 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH v2] tcp: bug fix in proportional rate reduction.

On Tue, May 21, 2013 at 6:12 PM, Nandita Dukkipati <nanditad@...gle.com> wrote:
> This patch is a fix for a bug triggering newly_acked_sacked < 0
> in tcp_ack(.).
>
> The bug is triggered by sacked_out decreasing relative to prior_sacked,
> but packets_out remaining the same as pior_packets. This is because the
> snapshot of prior_packets is taken after tcp_sacktag_write_queue() while
> prior_sacked is captured before tcp_sacktag_write_queue(). The problem
> is: tcp_sacktag_write_queue (tcp_match_skb_to_sack() -> tcp_fragment)
> adjusts the pcount for packets_out and sacked_out (MSS change or other
> reason). As a result, this delta in pcount is reflected in
> (prior_sacked - sacked_out) but not in (prior_packets - packets_out).
>
> This patch does the following:
> 1) initializes prior_packets at the start of tcp_ack() so as to
> capture the delta in packets_out created by tcp_fragment.
> 2) introduces a new "previous_packets_out" variable that snapshots
> packets_out right before tcp_clean_rtx_queue, so pkts_acked can be
> correctly computed as before.
> 3) Computes pkts_acked using previous_packets_out, and computes
> newly_acked_sacked using prior_packets.
>
> Signed-off-by: Nandita Dukkipati <nanditad@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>
> ---
> Changelog since v1:
> - Compute newly_acked_sacked using prior_packets.
>
>  net/ipv4/tcp_input.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index d7d3694..8230cd6 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2679,8 +2679,8 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
>   * tcp_xmit_retransmit_queue().
>   */
>  static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
> -                                 int prior_sacked, bool is_dupack,
> -                                 int flag)
> +                                 int prior_sacked, int prior_packets,
> +                                 bool is_dupack, int flag)
>  {
>         struct inet_connection_sock *icsk = inet_csk(sk);
>         struct tcp_sock *tp = tcp_sk(sk);
> @@ -2740,7 +2740,8 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
>                                 tcp_add_reno_sack(sk);
>                 } else
>                         do_lost = tcp_try_undo_partial(sk, pkts_acked);
> -               newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;
> +               newly_acked_sacked = prior_packets - tp->packets_out +
> +                                    tp->sacked_out - prior_sacked;
>                 break;
>         case TCP_CA_Loss:
>                 tcp_process_loss(sk, flag, is_dupack);
> @@ -2754,7 +2755,8 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
>                         if (is_dupack)
>                                 tcp_add_reno_sack(sk);
>                 }
> -               newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;
> +               newly_acked_sacked = prior_packets - tp->packets_out +
> +                                    tp->sacked_out - prior_sacked;
>
>                 if (icsk->icsk_ca_state <= TCP_CA_Disorder)
>                         tcp_try_undo_dsack(sk);
> @@ -3265,9 +3267,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>         bool is_dupack = false;
>         u32 prior_in_flight;
>         u32 prior_fackets;
> -       int prior_packets;
> +       int prior_packets = tp->packets_out;
>         int prior_sacked = tp->sacked_out;
>         int pkts_acked = 0;
> +       int previous_packets_out = 0;
>
>         /* If the ack is older than previous acks
>          * then we can probably ignore it.
> @@ -3338,14 +3341,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>         sk->sk_err_soft = 0;
>         icsk->icsk_probes_out = 0;
>         tp->rcv_tstamp = tcp_time_stamp;
> -       prior_packets = tp->packets_out;
>         if (!prior_packets)
>                 goto no_queue;
>
>         /* See if we can take anything off of the retransmit queue. */
> +       previous_packets_out = tp->packets_out;
>         flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);
>
> -       pkts_acked = prior_packets - tp->packets_out;
> +       pkts_acked = previous_packets_out - tp->packets_out;
>
>         if (tcp_ack_is_dubious(sk, flag)) {
>                 /* Advance CWND, if state allows this. */
> @@ -3353,7 +3356,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>                         tcp_cong_avoid(sk, ack, prior_in_flight);
>                 is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
>                 tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
> -                                     is_dupack, flag);
> +                                     prior_packets, is_dupack, flag);
>         } else {
>                 if (flag & FLAG_DATA_ACKED)
>                         tcp_cong_avoid(sk, ack, prior_in_flight);
> @@ -3376,7 +3379,7 @@ no_queue:
>         /* If data was DSACKed, see if we can undo a cwnd reduction. */
>         if (flag & FLAG_DSACKING_ACK)
>                 tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
> -                                     is_dupack, flag);
> +                                     prior_packets, is_dupack, flag);
>         /* If this ack opens up a zero window, clear backoff.  It was
>          * being used to time the probes, and is probably far higher than
>          * it needs to be for normal retransmission.
> @@ -3399,7 +3402,7 @@ old_ack:
>         if (TCP_SKB_CB(skb)->sacked) {
>                 flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
>                 tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
> -                                     is_dupack, flag);
> +                                     prior_packets, is_dupack, flag);
>         }
>
>         SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
> --
> 1.8.2.1
>
--
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