On Thu, Dec 01, 2016 at 03:29:49PM -0500, David Miller wrote:
> From: Marcelo Ricardo Leitner <[email protected]>
> 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 <[email protected]>
> > Signed-off-by: Marcelo Ricardo Leitner <[email protected]>
>
> 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.
>
> 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.
It's not avaliable anymore.. It's NULLified before we get there:
tcp_v4_rcv() (same for v6)
{
...
skb->dev = NULL;
...
if (!sock_owned_by_user(sk)) {
if (!tcp_prequeue(sk, skb))
ret = tcp_v4_do_rcv(sk, skb);
} else if (tcp_add_backlog(sk, skb)) {
...
}
I'll update the msg as above and post v2.
Thanks,
Marcelo
