tcp: avoid slow start during fast recovery on new losses

During TCP fast recovery, the congestion control in charge is by
default the Proportional Rate Reduction (PRR) unless the congestion
control module specified otherwise (e.g. BBR).

Previously when tcp_packets_in_flight() is below snd_ssthresh PRR
would slow start upon receiving an ACK that
   1) cumulatively acknowledges retransmitted data
   and
   2) does not detect further lost retransmission

Such conditions indicate the repair is in good steady progress
after the first round trip of recovery. Otherwise PRR adopts the
packet conservation principle to send only the amount that was
newly delivered (indicated by this ACK).

This patch generalizes the previous design principle to include
also the newly sent data beside retransmission: as long as
the delivery is making good progress, both retransmission and
new data should be accounted to make PRR more cautious in slow
starting.
Suggested-by: Matt Mathis <mattmathis@google.com>
Suggested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201031013412.1973112-1-ycheng@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -386,7 +386,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 int tcp_child_process(struct sock *parent, struct sock *child,
 		      struct sk_buff *skb);
 void tcp_enter_loss(struct sock *sk);
-void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int flag);
+void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, int flag);
 void tcp_clear_retrans(struct tcp_sock *tp);
 void tcp_update_metrics(struct sock *sk);
 void tcp_init_metrics(struct sock *sk);

net/ipv4/tcp_input.c

@@ -2546,7 +2546,7 @@ static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo)
  *   1) If the packets in flight is larger than ssthresh, PRR spreads the
  *	cwnd reductions across a full RTT.
  *   2) Otherwise PRR uses packet conservation to send as much as delivered.
- *      But when the retransmits are acked without further losses, PRR
+ *      But when SND_UNA is acked without further losses,
  *      slow starts cwnd up to ssthresh to speed up the recovery.
  */
 static void tcp_init_cwnd_reduction(struct sock *sk)
@@ -2563,7 +2563,7 @@ static void tcp_init_cwnd_reduction(struct sock *sk)
 	tcp_ecn_queue_cwr(tp);
 }
 
-void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int flag)
+void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, int flag)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	int sndcnt = 0;
@@ -2577,8 +2577,7 @@ void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int flag)
 		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
 			       tp->prior_cwnd - 1;
 		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
-	} else if ((flag & (FLAG_RETRANS_DATA_ACKED | FLAG_LOST_RETRANS)) ==
-		   FLAG_RETRANS_DATA_ACKED) {
+	} else if (flag & FLAG_SND_UNA_ADVANCED && !newly_lost) {
 		sndcnt = min_t(int, delta,
 			       max_t(int, tp->prr_delivered - tp->prr_out,
 				     newly_acked_sacked) + 1);
@@ -3419,7 +3418,7 @@ static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
 
 	if (tcp_in_cwnd_reduction(sk)) {
 		/* Reduce cwnd if state mandates */
-		tcp_cwnd_reduction(sk, acked_sacked, flag);
+		tcp_cwnd_reduction(sk, acked_sacked, rs->losses, flag);
 	} else if (tcp_may_raise_cwnd(sk, flag)) {
 		/* Advance cwnd if state allows */
 		tcp_cong_avoid(sk, ack, acked_sacked);

net/ipv4/tcp_recovery.c

@@ -153,6 +153,7 @@ void tcp_rack_reo_timeout(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 timeout, prior_inflight;
+	u32 lost = tp->lost;
 
 	prior_inflight = tcp_packets_in_flight(tp);
 	tcp_rack_detect_loss(sk, &timeout);
@@ -160,7 +161,7 @@ void tcp_rack_reo_timeout(struct sock *sk)
 		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
 			tcp_enter_recovery(sk, false);
 			if (!inet_csk(sk)->icsk_ca_ops->cong_control)
-				tcp_cwnd_reduction(sk, 1, 0);
+				tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
 		}
 		tcp_xmit_retransmit_queue(sk);
 	}