tcp: remove mss check in tcp_select_initial_window()

tracywwnj · davem330 · commit c36207bd8774 · 2018-04-27T14:05:36.000-04:00
In tcp_select_initial_window(), we only set rcv_wnd to
tcp_default_init_rwnd() if current mss &gt; (1 &lt;&lt; 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 &lt;weiwan@google.com&gt;
Acked-by: Eric Dumazet &lt;edumazet@google.com&gt;
Acked-by: Soheil Hassas Yeganeh &lt;soheil@google.com&gt;
Acked-by: Yuchung Cheng &lt;ycheng@google.com&gt;
Acked-by: Neal Cardwell &lt;ncardwell@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git 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);

Original file line number	Diff line number	Diff line change
`@@ -229,11 +229,9 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,`
`229`	`229`	`}`
`230`	`230`	`}`
`231`	`231`
`232`		`- if (mss > (1 << *rcv_wscale)) {`
`233`		`- if (!init_rcv_wnd) /* Use default unless specified otherwise */`
`234`		`- init_rcv_wnd = tcp_default_init_rwnd(mss);`
`235`		`- rcv_wnd = min(rcv_wnd, init_rcv_wnd * mss);`
`236`		`- }`
	`232`	`+ if (!init_rcv_wnd) /* Use default unless specified otherwise */`
	`233`	`+ init_rcv_wnd = tcp_default_init_rwnd(mss);`
	`234`	`+ rcv_wnd = min(rcv_wnd, init_rcv_wnd * mss);`
`237`	`235`
`238`	`236`	`/* Set the clamp no higher than max representable value */`
`239`	`237`	`(window_clamp) = min_t(__u32, U16_MAX << (rcv_wscale), *window_clamp);`