From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <netdev-owner@vger.kernel.org>
Received: from shards.monkeyblade.net ([184.105.139.130]:50748 "EHLO
        shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750725AbeBVTQt (ORCPT
        <rfc822;netdev@vger.kernel.org>); Thu, 22 Feb 2018 14:16:49 -0500
Date: Thu, 22 Feb 2018 14:16:47 -0500 (EST)
Message-Id: <20180222.141647.1474414618530735347.davem@davemloft.net>
To: eric.dumazet@gmail.com
Cc: oleksandr@natalenko.name, edumazet@google.com,
        netdev@vger.kernel.org, soheil@google.com, ncardwell@google.com
Subject: Re: [PATCH net] tcp_bbr: better deal with suboptimal GSO
From: David Miller <davem@davemloft.net>
In-Reply-To: <1519224183.55655.40.camel@gmail.com>
References: <2638137.lzCXOlOYsz@natalenko.name>
        <1519168897.55655.31.camel@gmail.com>
        <1519224183.55655.40.camel@gmail.com>
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 21 Feb 2018 06:43:03 -0800

> From: Eric Dumazet <edumazet@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.
 . ..
> Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>


Applied and queued up for -stable, thanks Eric.