On Thu, Dec 01, 2016 at 03:29:49PM -0500, David Miller wrote:
> From: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com>
> Date: Wed, 30 Nov 2016 11:14:32 -0200
>
> > There have been some reports lately about TCP connection stalls caused
> > by NIC drivers that aren't setting gso_size on aggregated packets on rx
> > path. This causes TCP to assume that the MSS is actually the size of the
> > aggregated packet, which is invalid.
> >
> > Although the proper fix is to be done at each driver, it's often hard
> > and cumbersome for one to debug, come to such root cause and report/fix
> > it.
> >
> > This patch amends this situation in two ways. First, it adds a warning
> > on when this situation occurs, so it gives a hint to those trying to
> > debug this. It also limit the maximum probed MSS to the adverised MSS,
> > as it should never be any higher than that.
> >
> > The result is that the connection may not have the best performance ever
> > but it shouldn't stall, and the admin will have a hint on what to look
> > for.
> >
> > Tested with virtio by forcing gso_size to 0.
> >
> > Cc: Jonathan Maxwell <jmaxwel...@gmail.com>
> > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com>
>
> I totally agree with this change, however I think the warning message can
> be improved in two ways:
>
> > len = skb_shinfo(skb)->gso_size ? : skb->len;
> > if (len >= icsk->icsk_ack.rcv_mss) {
> > - icsk->icsk_ack.rcv_mss = len;
> > + icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
> > + tcp_sk(sk)->advmss);
> > + if (icsk->icsk_ack.rcv_mss != len)
> > + pr_warn_once("Seems your NIC driver is doing bad RX
> > acceleration. TCP performance may be compromised.\n");
>
> We know it's a bad GRO implementation that causes this so let's be specific
> in the
> message, perhaps something like:
>
> Driver has suspect GRO implementation, TCP performance may be
> compromised.
Okay.
>
> Also, we have skb->dev available here most likely, so prefixing the message
> with
> skb->dev->name would make analyzing this situation even easier for someone
> hitting
> this.
Nice, yes.
And this skb is mostly non-forwardable as it's bigger than the MTU,
so if someone is using net namespaces and this skb would be routed
through some veth interfaces, it would give a false hint then, but
shouldn't happen. Unless it would fit (a larger) veth mtu, but still,
one probably will simplify things up to debug this.
>
> I'm not certain if an skb->dev==NULL check is necessary here or not, but it is
> definitely something you need to consider.
>
> Thanks!
>
Will check. Thanks!
Marcelo
