tcp: move cwnd reduction after recovery state procesing

Currently the cwnd is reduced and increased in various different
places. The reduction happens in various places in the recovery
state processing (tcp_fastretrans_alert) while the increase
happens afterward.

A better sequence is to identify lost packets and update
the congestion control state (icsk_ca_state) first. Then base
on the new state, up/down the cwnd in one central place. It's
more clear to reason cwnd changes.

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>

1 files changed, 28 insertions, 32 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 84a4ab9c05d1..dc810df53e90 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2471,14 +2471,12 @@ static void tcp_init_cwnd_reduction(struct sock *sk)
         tcp_ecn_queue_cwr(tp);
 }
 
-static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
-                               int fast_rexmit, int flag)
+static void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked,
+                               int flag)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         int sndcnt = 0;
         int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
-        int newly_acked_sacked = prior_unsacked -
-                                 (tp->packets_out - tp->sacked_out);
 
         if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
                 return;
@@ -2496,7 +2494,8 @@ static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
         } else {
                 sndcnt = min(delta, newly_acked_sacked);
         }
-        sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
+        /* Force a fast retransmit upon entering fast recovery */
+        sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
         tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
 }
 
@@ -2541,7 +2540,7 @@ static void tcp_try_keep_open(struct sock *sk)
         }
 }
 
-static void tcp_try_to_open(struct sock *sk, int flag, const int prior_unsacked)
+static void tcp_try_to_open(struct sock *sk, int flag)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
@@ -2555,8 +2554,6 @@ static void tcp_try_to_open(struct sock *sk, int flag, const int prior_unsacked)
 
         if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
                 tcp_try_keep_open(sk);
-        } else {
-                tcp_cwnd_reduction(sk, prior_unsacked, 0, flag);
         }
 }
 
@@ -2720,8 +2717,7 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
 }
 
 /* Undo during fast recovery after partial ACK. */
-static bool tcp_try_undo_partial(struct sock *sk, const int acked,
-                                 const int prior_unsacked, int flag)
+static bool tcp_try_undo_partial(struct sock *sk, const int acked)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
@@ -2736,10 +2732,8 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
                  * can undo. Otherwise we clock out new packets but do not
                  * mark more packets lost or retransmit more.
                  */
-                if (tp->retrans_out) {
-                        tcp_cwnd_reduction(sk, prior_unsacked, 0, flag);
+                if (tp->retrans_out)
                         return true;
-                }
 
                 if (!tcp_any_retrans_done(sk))
                         tp->retrans_stamp = 0;
@@ -2758,21 +2752,21 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
  * taking into account both packets sitting in receiver's buffer and
  * packets lost by network.
  *
- * Besides that it does CWND reduction, when packet loss is detected
- * and changes state of machine.
+ * Besides that it updates the congestion state when packet loss or ECN
+ * is detected. But it does not reduce the cwnd, it is done by the
+ * congestion control later.
  *
  * It does _not_ decide what to send, it is made in function
  * tcp_xmit_retransmit_queue().
  */
 static void tcp_fastretrans_alert(struct sock *sk, const int acked,
-                                  const int prior_unsacked,
-                                  bool is_dupack, int flag, int *rexmit)
+                                  bool is_dupack, int *ack_flag, int *rexmit)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
+        int fast_rexmit = 0, flag = *ack_flag;
         bool do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
                                     (tcp_fackets_out(tp) > tp->reordering));
-        int fast_rexmit = 0;
 
         if (WARN_ON(!tp->packets_out && tp->sacked_out))
                 tp->sacked_out = 0;
@@ -2819,8 +2813,10 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
 
         /* Use RACK to detect loss */
         if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS &&
-            tcp_rack_mark_lost(sk))
+            tcp_rack_mark_lost(sk)) {
                 flag |= FLAG_LOST_RETRANS;
+                *ack_flag |= FLAG_LOST_RETRANS;
+        }
 
         /* E. Process state. */
         switch (icsk->icsk_ca_state) {
@@ -2829,7 +2825,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
                         if (tcp_is_reno(tp) && is_dupack)
                                 tcp_add_reno_sack(sk);
                 } else {
-                        if (tcp_try_undo_partial(sk, acked, prior_unsacked, flag))
+                        if (tcp_try_undo_partial(sk, acked))
                                 return;
                         /* Partial ACK arrived. Force fast retransmit. */
                         do_lost = tcp_is_reno(tp) ||
@@ -2858,7 +2854,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
                         tcp_try_undo_dsack(sk);
 
                 if (!tcp_time_to_recover(sk, flag)) {
-                        tcp_try_to_open(sk, flag, prior_unsacked);
+                        tcp_try_to_open(sk, flag);
                         return;
                 }
 
@@ -2880,7 +2876,6 @@ 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);
         *rexmit = REXMIT_LOST;
 }
 
@@ -3306,9 +3301,6 @@ static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
 /* Decide wheather to run the increase function of congestion control. */
 static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
 {
-        if (tcp_in_cwnd_reduction(sk))
-                return false;
-
         /* If reordering is high then always grow cwnd whenever data is
          * delivered regardless of its ordering. Otherwise stay conservative
          * and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
@@ -3551,6 +3543,7 @@ 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 acked_sacked; /* Number of packets newly acked or sacked */
         int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
 
         sack_state.first_sackt.v64 = 0;
@@ -3647,15 +3640,20 @@ 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, &rexmit);
+                tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
         }
         if (tp->tlp_high_seq)
                 tcp_process_tlp_ack(sk, ack, flag);
 
+        acked_sacked = prior_unsacked - (tp->packets_out - tp->sacked_out);
         /* Advance cwnd if state allows */
-        if (tcp_may_raise_cwnd(sk, flag))
+        if (tcp_in_cwnd_reduction(sk)) {
+                /* Reduce cwnd if state mandates */
+                tcp_cwnd_reduction(sk, acked_sacked, flag);
+        } else if (tcp_may_raise_cwnd(sk, flag)) {
+                /* Advance cwnd if state allows */
                 tcp_cong_avoid(sk, ack, acked);
+        }
 
         if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
                 struct dst_entry *dst = __sk_dst_get(sk);
@@ -3672,8 +3670,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 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, &rexmit);
+                tcp_fastretrans_alert(sk, acked, 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.
@@ -3696,8 +3693,7 @@ old_ack:
         if (TCP_SKB_CB(skb)->sacked) {
                 flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                                                 &sack_state);
-                tcp_fastretrans_alert(sk, acked, prior_unsacked,
-                                      is_dupack, flag, &rexmit);
+                tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
                 tcp_xmit_recovery(sk, rexmit);
         }