netdev - Re: "TCP: eth0: Driver has suspect GRO implementation, TCP performance may be compromised." message with "ethtool -K eth0 gro off"

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Date:   Sun, 19 Mar 2017 12:20:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        netdev@...r.kernel.org
Subject: Re: "TCP: eth0: Driver has suspect GRO implementation, TCP
 performance may be compromised." message with "ethtool -K eth0 gro off"

On Sun, 2017-03-19 at 13:14 +0100, Markus Trippelsdorf wrote:
> On 2017.02.06 at 19:12 -0200, Marcelo Ricardo Leitner wrote:
> > On Fri, Feb 03, 2017 at 06:47:33AM -0800, Eric Dumazet wrote:
> > > On Fri, 2017-02-03 at 12:28 -0200, Marcelo Ricardo Leitner wrote:
> > > 
> > > > Aren't you mixing the endpoints here? MSS is the largest amount of data
> > > > that the peer can receive in a single segment, and not how much it will
> > > > send. For the sending part, that depends on what the other peer
> > > > announced, and we can have 2 different MSS in a single connection, one
> > > > for each peer.
> > > > 
> > > > If a peer later wants to send larger segments, it can, but it must
> > > > respect the mss advertised by the other peer during handshake.
> > > > 
> > > 
> > > I am not mixing endpoints, you are.
> > > 
> > > If you need to be convinced, please grab :
> > > https://patchwork.ozlabs.org/patch/723028/
> > > 
> > > And just watch "ss -temoi ..." 
> > 
> > I still don't get it, but I also hit the warning on my laptop, using
> > iwlwifi. Not sure what I did in order to trigger it, it was by accident.
> 
> After many weeks without any warning, I've hit the issue again today:
> 
>  TCP: eth0: Driver has suspect GRO implementation, TCP performance may be compromised. rcv_mss:1448 advmss:1448 len:1460
> 

It is very possible the sender suddenly forgot to use TCP timestamps.

This warning is a hint, and can not assume senders are not dumb.