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Date:      Thu, 3 Sep 2020 08:41:39 +0000 (UTC)
From:      Richard Scheffenegger <rscheff@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org
Subject:   svn commit: r365293 - stable/12/sys/netinet/cc
Message-ID:  <202009030841.0838fdO3040676@repo.freebsd.org>
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Author: rscheff
Date: Thu Sep  3 08:41:38 2020
New Revision: 365293
URL: https://svnweb.freebsd.org/changeset/base/365293

Log:
  MFC r364196: TCP Cubic: After leaving slowstart fix unintended cwnd jump.
  
  Initializing K to zero in D23655 introduced a miscalculation,
  where cwnd would suddenly jump to cwnd_max instead of gradually
  increasing, after leaving slow-start.
  
  Properly calculating K instead of resetting it to zero resolves
  this issue. Also making sure, that cwnd is recalculated at the
  earliest opportunity once slow-start is over.
  
  Reported by:	chengc_netapp.com
  Reviewed by:	chengc_netapp.com, tuexen (mentor), rgrimes (mentor)
  Approved by:	tuexen (mentor), rgrimes (mentor)
  MFC after:	3 weeks
  Sponsored by:	NetApp, Inc.
  Differential Revision:	https://reviews.freebsd.org/D25746

Modified:
  stable/12/sys/netinet/cc/cc_cubic.c
Directory Properties:
  stable/12/   (props changed)

Modified: stable/12/sys/netinet/cc/cc_cubic.c
==============================================================================
--- stable/12/sys/netinet/cc/cc_cubic.c	Thu Sep  3 08:38:04 2020	(r365292)
+++ stable/12/sys/netinet/cc/cc_cubic.c	Thu Sep  3 08:41:38 2020	(r365293)
@@ -132,19 +132,29 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
 
 	/*
 	 * Regular ACK and we're not in cong/fast recovery and we're cwnd
-	 * limited and we're either not doing ABC or are slow starting or are
-	 * doing ABC and we've sent a cwnd's worth of bytes.
+	 * limited and we're either not doing ABC or are just coming out
+	 * from slow-start or were application limited or are slow starting
+	 * or are doing ABC and we've sent a cwnd's worth of bytes.
 	 */
 	if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
 	    (ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
+	    (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART | CUBICFLAG_IN_APPLIMIT)) ||
 	    CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
-	    (V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
+	    (V_tcp_do_rfc3465 && (ccv->flags & CCF_ABC_SENTAWND)))) {
 		 /* Use the logic in NewReno ack_received() for slow start. */
 		if (CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
 		    cubic_data->min_rtt_ticks == TCPTV_SRTTBASE) {
 			cubic_data->flags |= CUBICFLAG_IN_SLOWSTART;
 			newreno_cc_algo.ack_received(ccv, type);
 		} else {
+			if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
+						 CUBICFLAG_IN_APPLIMIT)) {
+				cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
+						       CUBICFLAG_IN_APPLIMIT);
+				cubic_data->t_last_cong = ticks;
+				cubic_data->K = cubic_k(cubic_data->max_cwnd /
+							CCV(ccv, t_maxseg));
+			}
 			if ((ticks_since_cong =
 			    ticks - cubic_data->t_last_cong) < 0) {
 				/*
@@ -152,14 +162,6 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
 				 */
 				ticks_since_cong = INT_MAX;
 				cubic_data->t_last_cong = ticks - INT_MAX;
-			}
-
-			if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
-						 CUBICFLAG_IN_APPLIMIT)) {
-				cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
-						       CUBICFLAG_IN_APPLIMIT);
-				cubic_data->t_last_cong = ticks;
-				cubic_data->K = 0;
 			}
 			/*
 			 * The mean RTT is used to best reflect the equations in

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