On Wed, Feb 21, 2018 at 10:14 AM Neal Cardwell <ncardw...@google.com> wrote:
> On Wed, Feb 21, 2018 at 9:43 AM, Eric Dumazet <eric.duma...@gmail.com>
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(-)
> Acked-by: Neal Cardwell <ncardw...@google.com>
Acked-by: Soheil Hassas Yeganeh <soh...@google.com>
Thank you Eric for the nice patch!
