tcp: fix the timid additive increase on stretch ACKs

nealcardwell · davem330 · commit 814d488c6126 · 2015-01-28T22:18:37.000-08:00
tcp_cong_avoid_ai() was too timid (snd_cwnd increased too slowly) on
"stretch ACKs" -- cases where the receiver ACKed more than 1 packet in
a single ACK. For example, suppose w is 10 and we get a stretch ACK
for 20 packets, so acked is 20. We ought to increase snd_cwnd by 2
(since acked/w = 20/10 = 2), but instead we were only increasing cwnd
by 1. This patch fixes that behavior.

Reported-by: Eyal Perry &lt;eyalpe@mellanox.com&gt;
Signed-off-by: Neal Cardwell &lt;ncardwell@google.com&gt;
Signed-off-by: Yuchung Cheng &lt;ycheng@google.com&gt;
Signed-off-by: Eric Dumazet &lt;edumazet@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
@@ -304,16 +304,19 @@ u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
 }
 EXPORT_SYMBOL_GPL(tcp_slow_start);
 
-/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */
+/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
+ * for every packet that was ACKed.
+ */
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
 {
+	tp->snd_cwnd_cnt += acked;
 	if (tp->snd_cwnd_cnt >= w) {
-		if (tp->snd_cwnd < tp->snd_cwnd_clamp)
-			tp->snd_cwnd++;
-		tp->snd_cwnd_cnt = 0;
-	} else {
-		tp->snd_cwnd_cnt += acked;
+		u32 delta = tp->snd_cwnd_cnt / w;
+
+		tp->snd_cwnd_cnt -= delta * w;
+		tp->snd_cwnd += delta;
 	}
+	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
 }
 EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);
 

Original file line number	Diff line number	Diff line change
`@@ -304,16 +304,19 @@ u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)`
`304`	`304`	`}`
`305`	`305`	`EXPORT_SYMBOL_GPL(tcp_slow_start);`
`306`	`306`
`307`		`-/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */`
	`307`	`+/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),`
	`308`	`+ * for every packet that was ACKed.`
	`309`	`+ */`
`308`	`310`	`void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)`
`309`	`311`	`{`
	`312`	`+ tp->snd_cwnd_cnt += acked;`
`310`	`313`	`if (tp->snd_cwnd_cnt >= w) {`
`311`		`- if (tp->snd_cwnd < tp->snd_cwnd_clamp)`
`312`		`- tp->snd_cwnd++;`
`313`		`- tp->snd_cwnd_cnt = 0;`
`314`		`- } else {`
`315`		`- tp->snd_cwnd_cnt += acked;`
	`314`	`+ u32 delta = tp->snd_cwnd_cnt / w;`
	`315`	`+`
	`316`	`+ tp->snd_cwnd_cnt -= delta * w;`
	`317`	`+ tp->snd_cwnd += delta;`
`316`	`318`	`}`
	`319`	`+ tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);`
`317`	`320`	`}`
`318`	`321`	`EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);`
`319`	`322`