tcp: retransmit after recovery processing and congestion control

The retransmission and F-RTO transmission currently happen inside
recovery state processing (tcp_fastretrans_alert) but before
congestion control.  This refactoring moves the logic after both
s.t. we can determine how much to send (cwnd) before deciding what to
send.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8194a25..84a4ab9 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -126,6 +126,10 @@ int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;
 #define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
 #define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))
 
+#define REXMIT_NONE    0 /* no loss recovery to do */
+#define REXMIT_LOST    1 /* retransmit packets marked lost */
+#define REXMIT_NEW     2 /* FRTO-style transmit of unsent/new packets */
+
 /* Adapt the MSS value used to make delayed ack decision to the
  * real world.
  */
@@ -2662,7 +2666,8 @@ static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 /* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
  * recovered or spurious. Otherwise retransmits more on partial ACKs.
  */
-static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
+static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
+                            int *rexmit)
 {
        struct tcp_sock *tp = tcp_sk(sk);
        bool recovered = !before(tp->snd_una, tp->high_seq);
@@ -2684,10 +2689,15 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
                                tp->frto = 0; /* Step 3.a. loss was real */
                } else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
                        tp->high_seq = tp->snd_nxt;
-                       __tcp_push_pending_frames(sk, tcp_current_mss(sk),
-                                                 TCP_NAGLE_OFF);
-                       if (after(tp->snd_nxt, tp->high_seq))
-                               return; /* Step 2.b */
+                       /* Step 2.b. Try send new data (but deferred until cwnd
+                        * is updated in tcp_ack()). Otherwise fall back to
+                        * the conventional recovery.
+                        */
+                       if (tcp_send_head(sk) &&
+                           after(tcp_wnd_end(tp), tp->snd_nxt)) {
+                               *rexmit = REXMIT_NEW;
+                               return;
+                       }
                        tp->frto = 0;
                }
        }
@@ -2706,7 +2716,7 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
                else if (flag & FLAG_SND_UNA_ADVANCED)
                        tcp_reset_reno_sack(tp);
        }
-       tcp_xmit_retransmit_queue(sk);
+       *rexmit = REXMIT_LOST;
 }
 
 /* Undo during fast recovery after partial ACK. */
@@ -2756,7 +2766,7 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
  */
 static void tcp_fastretrans_alert(struct sock *sk, const int acked,
                                  const int prior_unsacked,
-                                 bool is_dupack, int flag)
+                                 bool is_dupack, int flag, int *rexmit)
 {
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
@@ -2831,7 +2841,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
                }
                break;
        case TCP_CA_Loss:
-               tcp_process_loss(sk, flag, is_dupack);
+               tcp_process_loss(sk, flag, is_dupack, rexmit);
                if (icsk->icsk_ca_state != TCP_CA_Open &&
                    !(flag & FLAG_LOST_RETRANS))
                        return;
@@ -2871,7 +2881,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
        if (do_lost)
                tcp_update_scoreboard(sk, fast_rexmit);
        tcp_cwnd_reduction(sk, prior_unsacked, fast_rexmit, flag);
-       tcp_xmit_retransmit_queue(sk);
+       *rexmit = REXMIT_LOST;
 }
 
 /* Kathleen Nichols' algorithm for tracking the minimum value of
@@ -3506,6 +3516,27 @@ static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
                icsk->icsk_ca_ops->in_ack_event(sk, flags);
 }
 
+/* Congestion control has updated the cwnd already. So if we're in
+ * loss recovery then now we do any new sends (for FRTO) or
+ * retransmits (for CA_Loss or CA_recovery) that make sense.
+ */
+static void tcp_xmit_recovery(struct sock *sk, int rexmit)
+{
+       struct tcp_sock *tp = tcp_sk(sk);
+
+       if (rexmit == REXMIT_NONE)
+               return;
+
+       if (unlikely(rexmit == 2)) {
+               __tcp_push_pending_frames(sk, tcp_current_mss(sk),
+                                         TCP_NAGLE_OFF);
+               if (after(tp->snd_nxt, tp->high_seq))
+                       return;
+               tp->frto = 0;
+       }
+       tcp_xmit_retransmit_queue(sk);
+}
+
 /* This routine deals with incoming acks, but not outgoing ones. */
 static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 {
@@ -3520,6 +3551,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
        int prior_packets = tp->packets_out;
        const int prior_unsacked = tp->packets_out - tp->sacked_out;
        int acked = 0; /* Number of packets newly acked */
+       int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
 
        sack_state.first_sackt.v64 = 0;
 
@@ -3616,7 +3648,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
        if (tcp_ack_is_dubious(sk, flag)) {
                is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
                tcp_fastretrans_alert(sk, acked, prior_unsacked,
-                                     is_dupack, flag);
+                                     is_dupack, flag, &rexmit);
        }
        if (tp->tlp_high_seq)
                tcp_process_tlp_ack(sk, ack, flag);
@@ -3634,13 +3666,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
        if (icsk->icsk_pending == ICSK_TIME_RETRANS)
                tcp_schedule_loss_probe(sk);
        tcp_update_pacing_rate(sk);
+       tcp_xmit_recovery(sk, rexmit);
        return 1;
 
 no_queue:
        /* If data was DSACKed, see if we can undo a cwnd reduction. */
        if (flag & FLAG_DSACKING_ACK)
                tcp_fastretrans_alert(sk, acked, prior_unsacked,
-                                     is_dupack, flag);
+                                     is_dupack, flag, &rexmit);
        /* 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.
@@ -3664,7 +3697,8 @@ old_ack:
                flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                                                &sack_state);
                tcp_fastretrans_alert(sk, acked, prior_unsacked,
-                                     is_dupack, flag);
+                                     is_dupack, flag, &rexmit);
+               tcp_xmit_recovery(sk, rexmit);
        }
 
        SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);