Patch "tcp_bbr: better deal with suboptimal GSO" has been added to the 4.9-stable tree

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 4.9-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-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Dumazet <[email protected]>
Date: Wed, 21 Feb 2018 06:43:03 -0800
Subject: tcp_bbr: better deal with suboptimal GSO

From: Eric Dumazet <[email protected]>

[ 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

---
 net/ipv4/tcp_output.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

@@ -1580,7 +1580,7 @@ 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);
 

@@ -1592,8 +1592,10 @@ 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, sysctl_tcp_min_tso_segs);
+	if (!tso_segs)
+		tso_segs = tcp_tso_autosize(sk, mss_now,
+					    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 [email protected] are

queue-4.9/tcp_bbr-better-deal-with-suboptimal-gso.patch