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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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