tcp: Add snd_wnd to TCP_INFO

Neal Cardwell mentioned that snd_wnd would be useful for diagnosing TCP
performance problems --
> (1) Usually when we're diagnosing TCP performance problems, we do so
> from the sender, since the sender makes most of the
> performance-critical decisions (cwnd, pacing, TSO size, TSQ, etc).
> From the sender-side the thing that would be most useful is to see
> tp->snd_wnd, the receive window that the receiver has advertised to
> the sender.

This serves the purpose of adding an additional __u32 to avoid the
would-be hole caused by the addition of the tcpi_rcvi_ooopack field.
Signed-off-by: Thomas Higdon <tph@fb.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -272,6 +272,10 @@ struct tcp_info {
 	__u32	tcpi_reord_seen;     /* reordering events seen */
 
 	__u32	tcpi_rcv_ooopack;    /* Out-of-order packets received */
+
+	__u32	tcpi_snd_wnd;	     /* peer's advertised receive window after
+				      * scaling (bytes)
+				      */
 };
 
 /* netlink attributes types for SCM_TIMESTAMPING_OPT_STATS */

net/ipv4/tcp.c

@@ -3297,6 +3297,7 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 	info->tcpi_dsack_dups = tp->dsack_dups;
 	info->tcpi_reord_seen = tp->reord_seen;
 	info->tcpi_rcv_ooopack = tp->rcv_ooopack;
+	info->tcpi_snd_wnd = tp->snd_wnd;
 	unlock_sock_fast(sk, slow);
 }
 EXPORT_SYMBOL_GPL(tcp_get_info);