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%3Ba=su...
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 stable@vger.kernel.org know about it.
From foo@baz Thu Mar 8 06:55:02 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 | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
--- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -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 edumazet@google.com are
queue-4.9/tcp_bbr-better-deal-with-suboptimal-gso.patch