[PATCH net-next] tcp: increase size at which tcp_bound_to_half_wnd bounds to > TCP_MSS_DEFAULT

[Seymour, Shane M]

In previous commit 01f83d69844d307be2aa6fea88b0e8fe5cbdb2f4
the following comments were added:

"When peer uses tiny windows, there is no use in packetizing to sub-MSS
pieces for the sake of SWS or making sure there are enough packets in
the pipe for fast recovery."

The test should be > TCP_MSS_DEFAULT not >= 512. This allows low end
devices that send an MSS of 536 (TCP_MSS_DEFAULT) to see better network
performance by sending it 536 bytes of data at a time instead of bounding
to half window size (268). Other network stacks work this way, e.g. HP-UX.

Signed-off-by: Shane Seymour <shane.seym...@hpe.com>
---
--- b/include/net/tcp.h 2016-06-23 20:59:14.521686048 -0500
+++ a/include/net/tcp.h 2016-06-15 17:19:21.964821477 -0500
@@ -589,7 +589,7 @@ static inline int tcp_bound_to_half_wnd(
         * On the other hand, for extremely large MSS devices, handling
         * smaller than MSS windows in this way does make sense.
         */
-       if (tp->max_window > TCP_MSS_DEFAULT)
+       if (tp->max_window >= 512)
                cutoff = (tp->max_window >> 1);
        else
                cutoff = tp->max_window;