tcp: Revert "tcp: remove CA_ACK_SLOWPATH"

This change was a followup to the header prediction removal,
so first revert this as a prerequisite to back out hp removal.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -910,8 +910,9 @@ enum tcp_ca_event {
 
 /* Information about inbound ACK, passed to cong_ops->in_ack_event() */
 enum tcp_ca_ack_event_flags {
-	CA_ACK_WIN_UPDATE	= (1 << 0),	/* ACK updated window */
-	CA_ACK_ECE		= (1 << 1),	/* ECE bit is set on ack */
+	CA_ACK_SLOWPATH		= (1 << 0),	/* In slow path processing */
+	CA_ACK_WIN_UPDATE	= (1 << 1),	/* ACK updated window */
+	CA_ACK_ECE		= (1 << 2),	/* ECE bit is set on ack */
 };
 
 /*

net/ipv4/tcp_input.c

@@ -3552,7 +3552,6 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	u32 lost = tp->lost;
 	int acked = 0; /* Number of packets newly acked */
 	int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
-	u32 ack_ev_flags = 0;
 
 	sack_state.first_sackt = 0;
 	sack_state.rate = &rs;
@@ -3593,6 +3592,9 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	if (flag & FLAG_UPDATE_TS_RECENT)
 		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
 
+	{
+		u32 ack_ev_flags = CA_ACK_SLOWPATH;
+
 		if (ack_seq != TCP_SKB_CB(skb)->end_seq)
 			flag |= FLAG_DATA;
 		else
@@ -3606,13 +3608,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 
 		if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
 			flag |= FLAG_ECE;
-		ack_ev_flags = CA_ACK_ECE;
+			ack_ev_flags |= CA_ACK_ECE;
 		}
 
 		if (flag & FLAG_WIN_UPDATE)
 			ack_ev_flags |= CA_ACK_WIN_UPDATE;
 
 		tcp_in_ack_event(sk, ack_ev_flags);
+	}
 
 	/* We passed data and got it acked, remove any soft error
 	 * log. Something worked...

net/ipv4/tcp_westwood.c

@@ -153,6 +153,24 @@ static inline void update_rtt_min(struct westwood *w)
 		w->rtt_min = min(w->rtt, w->rtt_min);
 }
 
+/*
+ * @westwood_fast_bw
+ * It is called when we are in fast path. In particular it is called when
+ * header prediction is successful. In such case in fact update is
+ * straight forward and doesn't need any particular care.
+ */
+static inline void westwood_fast_bw(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	struct westwood *w = inet_csk_ca(sk);
+
+	westwood_update_window(sk);
+
+	w->bk += tp->snd_una - w->snd_una;
+	w->snd_una = tp->snd_una;
+	update_rtt_min(w);
+}
+
 /*
  * @westwood_acked_count
  * This function evaluates cumul_ack for evaluating bk in case of
@@ -205,12 +223,17 @@ static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
 
 static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
 {
+	if (ack_flags & CA_ACK_SLOWPATH) {
 		struct westwood *w = inet_csk_ca(sk);
 
 		westwood_update_window(sk);
 		w->bk += westwood_acked_count(sk);
 
 		update_rtt_min(w);
+		return;
+	}
+
+	westwood_fast_bw(sk);
 }
 
 static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)