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, 6 insertions, 0 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 267164a1d559..4aa40ef02d32 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -148,6 +148,12 @@ struct tcp_sock {
         u16     gso_segs;       /* Max number of segs per GSO packet    */
 
 /*
+ *      Header prediction flags
+ *      0x5?10 << 16 + snd_wnd in net byte order
+ */
+        __be32  pred_flags;
+
+/*
  *      RFC793 variables by their proper names. This means you can
  *      read the code and the spec side by side (and laugh ...)
  *      See RFC793 and RFC1122. The RFC writes these in capitals.