netdev - [PATCH net-next] tcp: remove mss check in tcp_select_initial

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Date:   Thu, 26 Apr 2018 09:58:10 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Wei Wang <weiwan@...gle.com>
Subject: [PATCH net-next] tcp: remove mss check in tcp_select_initial_window()

From: Wei Wang <weiwan@...gle.com>

In tcp_select_initial_window(), we only set rcv_wnd to
tcp_default_init_rwnd() if current mss > (1 << wscale). Otherwise,
rcv_wnd is kept at the full receive space of the socket which is a
value way larger than tcp_default_init_rwnd().
With larger initial rcv_wnd value, receive buffer autotuning logic
takes longer to kick in and increase the receive buffer.

In a TCP throughput test where receiver has rmem[2] set to 125MB
(wscale is 11), we see the connection gets recvbuf limited at the
beginning of the connection and gets less throughput overall.

Signed-off-by: Wei Wang <weiwan@...gle.com>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_output.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 95feffb6d53f..d07c0dcc99aa 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -229,11 +229,9 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
 		}
 	}
 
-	if (mss > (1 << *rcv_wscale)) {
-		if (!init_rcv_wnd) /* Use default unless specified otherwise */
-			init_rcv_wnd = tcp_default_init_rwnd(mss);
-		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
-	}
+	if (!init_rcv_wnd) /* Use default unless specified otherwise */
+		init_rcv_wnd = tcp_default_init_rwnd(mss);
+	*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
 
 	/* Set the clamp no higher than max representable value */
 	(*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
-- 
2.17.0.484.g0c8726318c-goog