netdev - [PATCH net-next 2/5] tcp: fix stretch ACK bugs in Scalable

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Date:   Sat, 14 Mar 2020 00:47:21 +0800
From:   Pengcheng Yang <yangpc@...gsu.com>
To:     edumazet@...gle.com, davem@...emloft.net, ncardwell@...gle.com
Cc:     netdev@...r.kernel.org, Pengcheng Yang <yangpc@...gsu.com>
Subject: [PATCH net-next 2/5] 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@...gsu.com>
---
 net/ipv4/tcp_scalable.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_scalable.c b/net/ipv4/tcp_scalable.c
index 471571e..6cebf41 100644
--- a/net/ipv4/tcp_scalable.c
+++ b/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)
-- 
1.8.3.1