Patch "tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows" has been added to the 4.14-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows

to the 4.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tcp_cubic-fix-spurious-hystart-ack-train-detections-.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 7168ef436608d2408ca6a936dbbce2715d24509c
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Tue Nov 23 12:25:35 2021 -0800

    tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows
    
    [ Upstream commit 4e1fddc98d2585ddd4792b5e44433dcee7ece001 ]
    
    While testing BIG TCP patch series, I was expecting that TCP_RR workloads
    with 80KB requests/answers would send one 80KB TSO packet,
    then being received as a single GRO packet.
    
    It turns out this was not happening, and the root cause was that
    cubic Hystart ACK train was triggering after a few (2 or 3) rounds of RPC.
    
    Hystart was wrongly setting CWND/SSTHRESH to 30, while my RPC
    needed a budget of ~20 segments.
    
    Ideally these TCP_RR flows should not exit slow start.
    
    Cubic Hystart should reset itself at each round, instead of assuming
    every TCP flow is a bulk one.
    
    Note that even after this patch, Hystart can still trigger, depending
    on scheduling artifacts, but at a higher CWND/SSTHRESH threshold,
    keeping optimal TSO packet sizes.
    
    Tested:
    
    ip link set dev eth0 gro_ipv6_max_size 131072 gso_ipv6_max_size 131072
    nstat -n; netperf -H ... -t TCP_RR  -l 5  -- -r 80000,80000 -K cubic; nstat|egrep "Ip6InReceives|Hystart|Ip6OutRequests"
    
    Before:
    
       8605
    Ip6InReceives                   87541              0.0
    Ip6OutRequests                  129496             0.0
    TcpExtTCPHystartTrainDetect     1                  0.0
    TcpExtTCPHystartTrainCwnd       30                 0.0
    
    After:
    
      8760
    Ip6InReceives                   88514              0.0
    Ip6OutRequests                  87975              0.0
    
    Fixes: ae27e98a5152 ("[TCP] CUBIC v2.3")
    Co-developed-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
    Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Cc: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
    Cc: Yuchung Cheng <ycheng@xxxxxxxxxx>
    Cc: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20211123202535.1843771-1-eric.dumazet@xxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 8b5ba0a5cd386..93530bd332470 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -340,8 +340,6 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		return;
 
 	if (tcp_in_slow_start(tp)) {
-		if (hystart && after(ack, ca->end_seq))
-			bictcp_hystart_reset(sk);
 		acked = tcp_slow_start(tp, acked);
 		if (!acked)
 			return;
@@ -383,6 +381,9 @@ static void hystart_update(struct sock *sk, u32 delay)
 	if (ca->found & hystart_detect)
 		return;
 
+	if (after(tp->snd_una, ca->end_seq))
+		bictcp_hystart_reset(sk);
+
 	if (hystart_detect & HYSTART_ACK_TRAIN) {
 		u32 now = bictcp_clock();