netdev - [PATCH v2] tcp: cwnd does not increase in TCP YeAH

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Date:   Wed, 7 Sep 2016 10:49:36 -0700
From:   Artem Germanov <agermanov@...horfree.com>
To:     <netdev@...r.kernel.org>, <davem@...emloft.net>,
        <nanditad@...gle.com>, <edumazet@...gle.com>,
        <ncardwell@...gle.com>, <ycheng@...gle.com>,
        "Dmitry Adamushko" <d.adamushko@...horfree.com>
Subject: [PATCH v2] tcp: cwnd does not increase in TCP YeAH

Commit 76174004a0f19785a328f40388e87e982bbf69b9
(tcp: do not slow start when cwnd equals ssthresh )
introduced regression in TCP YeAH. Using 100ms delay 1% loss virtual 
ethernet link kernel 4.2 shows bandwidth ~500KB/s for single TCP 
connection and kernel 4.3 and above (including 4.8-rc4) shows bandwidth 
~100KB/s.
   That is caused by stalled cwnd when cwnd equals ssthresh. This patch 
fixes it by proper increasing cwnd in this case.

Signed-off-by: Artem Germanov <agermanov@...horfree.com>
Acked-by: Dmitry Adamushko <d.adamushko@...horfree.com>
---
diff --git a/net/ipv4/tcp_yeah.c b/net/ipv4/tcp_yeah.c
index 028eb04..9c5fc97 100644
--- a/net/ipv4/tcp_yeah.c
+++ b/net/ipv4/tcp_yeah.c
@@ -76,7 +76,7 @@ static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;

-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);

 	else if (!yeah->doing_reno_now) {