svn commit: r257367 - stable/10/sys/netinet

[Andre Oppermann]

Author: andre
Date: Tue Oct 29 21:00:54 2013
New Revision: 257367
URL: http://svnweb.freebsd.org/changeset/base/257367

Log:
  MFC r256920:
  
    The TCP delayed ACK logic isn't aware of LRO passing up large aggregated
    segments thinking it received only one segment. This causes it to enable
    the delay the ACK for 100ms to wait for another segment which may never
    come because all the data was received already.
  
    Doing delayed ACK for LRO segments is bogus for two reasons: a) it pushes
    us further away from acking every other packet; b) it introduces additional
    delay in responding to the sender.  The latter is especially bad because it
    is in the nature of LRO to aggregated all segments of a burst with no more
    coming until an ACK is sent back.
  
    Change the delayed ACK logic to detect LRO segments by being larger than
    the MSS for this connection and issuing an immediate ACK for them to keep
    the ACK clock ticking without interruption.
  
    Reported by:  julian, cperciva
    Tested by:    cperciva
    Reviewed by:  lstewart
  
  Approved by:    re (glebius)

Modified:
  stable/10/sys/netinet/tcp_input.c
Directory Properties:
  stable/10/sys/   (props changed)

Modified: stable/10/sys/netinet/tcp_input.c
==============================================================================
--- stable/10/sys/netinet/tcp_input.c	Tue Oct 29 20:53:09 2013	(r257366)
+++ stable/10/sys/netinet/tcp_input.c	Tue Oct 29 21:00:54 2013	(r257367)
@@ -508,10 +508,13 @@ do { \
  *	  the ack that opens up a 0-sized window and
  *		- delayed acks are enabled or
  *		- this is a half-synchronized T/TCP connection.
+ *	- the segment size is not larger than the MSS and LRO wasn't used
+ *	  for this segment.
  */
-#define DELAY_ACK(tp)							\
+#define DELAY_ACK(tp, tlen)						\
 	((!tcp_timer_active(tp, TT_DELACK) &&				\
 	    (tp->t_flags & TF_RXWIN0SENT) == 0) &&			\
+	    (tlen <= tp->t_maxopd) &&					\
 	    (V_tcp_delack_enabled || (tp->t_flags & TF_NEEDSYN)))
 
 /*
@@ -1863,7 +1866,7 @@ tcp_do_segment(struct mbuf *m, struct tc
 			}
 			/* NB: sorwakeup_locked() does an implicit unlock. */
 			sorwakeup_locked(so);
-			if (DELAY_ACK(tp)) {
+			if (DELAY_ACK(tp, tlen)) {
 				tp->t_flags |= TF_DELACK;
 			} else {
 				tp->t_flags |= TF_ACKNOW;
@@ -1954,7 +1957,7 @@ tcp_do_segment(struct mbuf *m, struct tc
 			 * If there's data, delay ACK; if there's also a FIN
 			 * ACKNOW will be turned on later.
 			 */
-			if (DELAY_ACK(tp) && tlen != 0)
+			if (DELAY_ACK(tp, tlen) && tlen != 0)
 				tcp_timer_activate(tp, TT_DELACK,
 				    tcp_delacktime);
 			else
@@ -2926,7 +2929,7 @@ dodata:							/* XXX */
 		if (th->th_seq == tp->rcv_nxt &&
 		    LIST_EMPTY(&tp->t_segq) &&
 		    TCPS_HAVEESTABLISHED(tp->t_state)) {
-			if (DELAY_ACK(tp))
+			if (DELAY_ACK(tp, tlen))
 				tp->t_flags |= TF_DELACK;
 			else
 				tp->t_flags |= TF_ACKNOW;