tcp: Revert "tcp: remove header prediction"

This reverts commit 45f119bf936b1f9f546a0b139c5b56f9bb2bdc78.

Eric Dumazet says:
  We found at Google a significant regression caused by
  45f119bf936b1f9f546a0b139c5b56f9bb2bdc78 tcp: remove header prediction

  In typical RPC  (TCP_RR), when a TCP socket receives data, we now call
  tcp_ack() while we used to not call it.

  This touches enough cache lines to cause a slowdown.

so problem does not seem to be HP removal itself but the tcp_ack()
call.  Therefore, it might be possible to remove HP after all, provided
one finds a way to elide tcp_ack for most cases.

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 3e0d19631534..5b6690d05abb 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -295,7 +295,9 @@ static u16 tcp_select_window(struct sock *sk)
         /* RFC1323 scaling applied */
         new_win >>= tp->rx_opt.rcv_wscale;
 
+        /* If we advertise zero window, disable fast path. */
         if (new_win == 0) {
+                tp->pred_flags = 0;
                 if (old_win)
                         NET_INC_STATS(sock_net(sk),
                                       LINUX_MIB_TCPTOZEROWINDOWADV);