netdev - Re: [PATCH net] tcp_bbr: better deal with suboptimal GSO

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Date:   Wed, 21 Feb 2018 10:14:02 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net] tcp_bbr: better deal with suboptimal GSO

On Wed, Feb 21, 2018 at 9:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> 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@...gle.com>
> Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>
> ---
>  net/ipv4/tcp_output.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal