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

From: Paolo Abeni <[email-masked]>
Date: Wed, 21 Feb 2018 16:01:12 +0100

Hi,

On Wed, 2018-02-21 at 06:43 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> 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 <[email-masked]>
> Reported-by: Oleksandr Natalenko <[email-masked]>
> ---
>  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