From: Thomas Graf <tgraf@redhat.com>
Subject: [RHEL5.1 BZ229909] high TCP latency with small packets
Date: Wed, 9 May 2007 20:29:12 +0200
Bugzilla: 229908
Message-Id: <20070509182912.GA14100@lsx.localdomain>
Changelog: [net] high TCP latency with small packets


Backport of upstream patch, no modification was necessary. Please ACK.

commit 1ef9696c909060ccdae3ade245ca88692b49285b
Author: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>

    [TCP]: Send ACKs each 2nd received segment.
    
    It does not affect either mss-sized connections (obviously) or
    connections controlled by Nagle (because there is only one small
    segment in flight).
    
    The idea is to record the fact that a small segment arrives on a
    connection, where one small segment has already been received and
    still not-ACKed. In this case ACK is forced after tcp_recvmsg() drains
    receive buffer.
    
    In other words, it is a "soft" each-2nd-segment ACK, which is enough
    to preserve ACK clock even when ABC is enabled.
    
    Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Index: linux-2.6.18.noarch/include/net/inet_connection_sock.h
===================================================================
--- linux-2.6.18.noarch.orig/include/net/inet_connection_sock.h	2007-05-09 16:48:10.000000000 +0200
+++ linux-2.6.18.noarch/include/net/inet_connection_sock.h	2007-05-09 20:24:01.000000000 +0200
@@ -147,7 +147,8 @@ extern struct sock *inet_csk_clone(struc
 enum inet_csk_ack_state_t {
 	ICSK_ACK_SCHED	= 1,
 	ICSK_ACK_TIMER  = 2,
-	ICSK_ACK_PUSHED = 4
+	ICSK_ACK_PUSHED = 4,
+	ICSK_ACK_PUSHED2 = 8
 };
 
 extern void inet_csk_init_xmit_timers(struct sock *sk,
Index: linux-2.6.18.noarch/net/ipv4/tcp.c
===================================================================
--- linux-2.6.18.noarch.orig/net/ipv4/tcp.c	2007-05-09 16:48:10.000000000 +0200
+++ linux-2.6.18.noarch/net/ipv4/tcp.c	2007-05-09 20:24:01.000000000 +0200
@@ -955,8 +955,11 @@ void tcp_cleanup_rbuf(struct sock *sk, i
 		     * receive buffer and there was a small segment
 		     * in queue.
 		     */
-		    (copied > 0 && (icsk->icsk_ack.pending & ICSK_ACK_PUSHED) &&
-		     !icsk->icsk_ack.pingpong && !atomic_read(&sk->sk_rmem_alloc)))
+		    (copied > 0 &&
+		     ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED2) ||
+		      ((icsk->icsk_ack.pending & ICSK_ACK_PUSHED) &&
+		       !icsk->icsk_ack.pingpong)) &&
+		      !atomic_read(&sk->sk_rmem_alloc)))
 			time_to_ack = 1;
 	}
 
Index: linux-2.6.18.noarch/net/ipv4/tcp_input.c
===================================================================
--- linux-2.6.18.noarch.orig/net/ipv4/tcp_input.c	2007-05-09 16:48:10.000000000 +0200
+++ linux-2.6.18.noarch/net/ipv4/tcp_input.c	2007-05-09 20:24:01.000000000 +0200
@@ -156,6 +156,8 @@ static void tcp_measure_rcv_mss(struct s
 				return;
 			}
 		}
+		if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
+			icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
 		icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
 	}
 }