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>

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 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	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);
 	}