From 350c9f484bde93ef229682eedd98cd5f74350f7f Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 21 Feb 2018 06:43:03 -0800 Subject: [PATCH] tcp_bbr: better deal with suboptimal GSO MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reported-by: Oleksandr Natalenko Acked-by: Neal Cardwell Acked-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- net/ipv4/tcp_output.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index b2bca373f8be..6818042cd8a9 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1730,7 +1730,7 @@ u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now, */ 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, unsigned int mss_now) 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 */ -- 2.30.2