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.
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.
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!
