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:   Fri, 3 Feb 2017 12:28:28 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Markus Trippelsdorf <markus@...ppelsdorf.de>,
        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 Fri, Feb 03, 2017 at 06:16:06AM -0800, Eric Dumazet wrote:
> On Fri, 2017-02-03 at 11:53 -0200, Marcelo Ricardo Leitner wrote:
> > On Fri, Feb 03, 2017 at 05:24:06AM -0800, Eric Dumazet wrote:
> > > On Fri, 2017-02-03 at 09:54 -0200, Marcelo Ricardo Leitner wrote:
> > > > On Thu, Feb 02, 2017 at 05:59:24AM -0800, Eric Dumazet wrote:
> > > > > On Thu, 2017-02-02 at 05:31 -0800, Eric Dumazet wrote:
> > > > > 
> > > > > > Anyway, I suspect the test is simply buggy ;)
> > > > > > 
> > > > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > > > > > index 41dcbd568cbe2403f2a9e659669afe462a42e228..5394a39fcce964a7fe7075b1531a8a1e05550a54 100644
> > > > > > --- a/net/ipv4/tcp_input.c
> > > > > > +++ b/net/ipv4/tcp_input.c
> > > > > > @@ -164,7 +164,7 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
> > > > > >  	if (len >= icsk->icsk_ack.rcv_mss) {
> > > > > >  		icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
> > > > > >  					       tcp_sk(sk)->advmss);
> > > > > > -		if (unlikely(icsk->icsk_ack.rcv_mss != len))
> > > > > > +		if (unlikely(icsk->icsk_ack.rcv_mss != len && skb_is_gso(skb)))
> > > > > >  			tcp_gro_dev_warn(sk, skb);
> > > > > >  	} else {
> > > > > >  		/* Otherwise, we make more careful check taking into account,
> > > > > 
> > > > > This wont really help.
> > > > > 
> > > > > Our tcp_sk(sk)->advmss can be lower than the MSS used by the remote
> > > > > peer.
> > > > > 
> > > > > ip ro add .... advmss 512
> > > > 
> > > > I don't follow. With a good driver, how can advmss be smaller than the
> > > > MSS used by the remote peer? Even with the route entry above, I get
> > > > segments just up to advmss, and no warning.
> > > > 
> > > 
> > > A TCP flow has two ends.
> > 
> > Indeed, though should be mostly about only one of them.
> > 
> > > 
> > > Common MTU = 1500
> > > 
> > > One can have advmss 500, the other one no advmss (or the standard 1460
> > > one)
> > 
> > Considering the rx side of peer A. Peer A advertises a given MSS to peer
> > B and should not receive any segment from peer B larger than so.
> > I'm failing to see how advmss can be smaller than the segment size just
> > received.
> 
> tcp_sk(sk)->advmss records what the peer announced during its SYN (or
> SYNACK) message, in the MSS option.
> 
> Nothing prevents the peer to change its mind later.
> 
> Eg starting with MSS 512, then switch later to sending packets of 1024
> or 1400 bytes.

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.

> 
> So the innocent NIC driver is not the problem here.
> 
>