Patch "tcp_bbr: better deal with suboptimal GSO" has been added to the 4.15-stable tree

[<gregkh@linuxfoundation.org>]

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

    tcp_bbr: better deal with suboptimal GSO

to the 4.15-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_bbr-better-deal-with-suboptimal-gso.patch
and it can be found in the queue-4.15 subdirectory.

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


>From foo@baz Tue Mar  6 19:02:56 PST 2018
From: Eric Dumazet <edumazet@google.com>
Date: Wed, 21 Feb 2018 06:43:03 -0800
Subject: tcp_bbr: better deal with suboptimal GSO

From: Eric Dumazet <edumazet@google.com>


[ Upstream commit 350c9f484bde93ef229682eedd98cd5f74350f7f ]

BBR uses tcp_tso_autosize() in an attempt to probe what would be the
burst sizes and to adjust cwnd in bbr_target_cwnd() with following
gold formula :

/* Allow enough full-sized skbs in flight to utilize end systems. */
cwnd += 3 * bbr->tso_segs_goal;

But GSO can be lacking or be constrained to very small
units (ip link set dev ... gso_max_segs 2)

What we really want is to have enough packets in flight so that both
GSO and GRO are efficient.

So in the case GSO is off or downgraded, we still want to have the same
number of packets in flight as if GSO/TSO was fully operational, so
that GRO can hopefully be working efficiently.

To fix this issue, we make tcp_tso_autosize() unaware of
sk->sk_gso_max_segs

Only tcp_tso_segs() has to enforce the gso_max_segs limit.

Tested:

ethtool -K eth0 tso off gso off
tc qd replace dev eth0 root pfifo_fast

Before patch:
for f in {1..5}; do ./super_netperf 1 -H lpaa24 -- -K bbr; done
    691  (ss -temoi shows cwnd is stuck around 6 )
    667
    651
    631
    517

After patch :
# for f in {1..5}; do ./super_netperf 1 -H lpaa24 -- -K bbr; done
   1733 (ss -temoi shows cwnd is around 386 )
   1778
   1746
   1781
   1718

Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_output.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1730,7 +1730,7 @@ u32 tcp_tso_autosize(const struct sock *
 	 */
 	segs = max_t(u32, bytes / mss_now, min_tso_segs);
 
-	return min_t(u32, segs, sk->sk_gso_max_segs);
+	return segs;
 }
 EXPORT_SYMBOL(tcp_tso_autosize);
 
@@ -1742,9 +1742,10 @@ static u32 tcp_tso_segs(struct sock *sk,
 	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
 	u32 tso_segs = ca_ops->tso_segs_goal ? ca_ops->tso_segs_goal(sk) : 0;
 
-	return tso_segs ? :
-		tcp_tso_autosize(sk, mss_now,
-				 sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);
+	if (!tso_segs)
+		tso_segs = tcp_tso_autosize(sk, mss_now,
+				sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);
+	return min_t(u32, tso_segs, sk->sk_gso_max_segs);
 }
 
 /* Returns the portion of skb which can be sent right away */


Patches currently in stable-queue which might be from edumazet@google.com are

queue-4.15/doc-change-the-min-default-value-of-tcp_wmem-tcp_rmem.patch
queue-4.15/tcp-purge-write-queue-upon-rst.patch
queue-4.15/net_sched-gen_estimator-fix-broken-estimators-based-on-percpu-stats.patch
queue-4.15/tcp_bbr-better-deal-with-suboptimal-gso.patch