tcp: fix stretch ACK bugs in Scalable

Change Scalable to properly handle stretch ACKs in additive
increase mode by passing in the count of ACKed packets to
tcp_cong_avoid_ai().

In addition, because we are now precisely accounting for
stretch ACKs, including delayed ACKs, we can now change
TCP_SCALABLE_AI_CNT to 100.
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_scalable.c

@@ -10,10 +10,9 @@
 #include <net/tcp.h>
 
 /* These factors derived from the recommended values in the aer:
- * .01 and and 7/8. We use 50 instead of 100 to account for
- * delayed ack.
+ * .01 and and 7/8.
  */
-#define TCP_SCALABLE_AI_CNT	50U
+#define TCP_SCALABLE_AI_CNT	100U
 #define TCP_SCALABLE_MD_SCALE	3
 
 static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
@@ -23,11 +22,13 @@ static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tcp_in_slow_start(tp))
-		tcp_slow_start(tp, acked);
-	else
-		tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT),
-				  1);
+	if (tcp_in_slow_start(tp)) {
+		acked = tcp_slow_start(tp, acked);
+		if (!acked)
+			return;
+	}
+	tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT),
+			  acked);
 }
 
 static u32 tcp_scalable_ssthresh(struct sock *sk)