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H A D | tcp_output.c | diff 350c9f484bde93ef229682eedd98cd5f74350f7f Wed Feb 21 15:43:03 CET 2018 Eric Dumazet <edumazet@google.com> tcp_bbr: better deal with suboptimal GSO
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>
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