Hi,
On Wed, 2018-02-21 at 06:43 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <eduma...@google.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 <eduma...@google.com>
> Reported-by: Oleksandr Natalenko <oleksa...@natalenko.name>
> ---
> 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
> b2bca373f8bee35267df49b5947a6793fed71a12..6818042cd8a9a1778f54637861647091afd9a769
> 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);
>
Very minor nit, why don't:
return max_t(u32, bytes / mss_now, min_tso_segs);
and drop the 'segs' local variable?
Thanks,
Paolo
