Re: [PATCH net-next] net: preserve sock reference when scrubbing the skb.

[Eric Dumazet <eric.dumazet@gmail.com>]

On 06/25/2018 09:15 PM, Cong Wang wrote:
> On Mon, Jun 25, 2018 at 8:59 AM Flavio Leitner <fbl@redhat.com> wrote:
>>
>> The sock reference is lost when scrubbing the packet and that breaks
>> TSQ (TCP Small Queues) and XPS (Transmit Packet Steering) causing
>> performance impacts of about 50% in a single TCP stream when crossing
>> network namespaces.
>>
>> XPS breaks because the queue mapping stored in the socket is not
>> available, so another random queue might be selected when the stack
>> needs to transmit something like a TCP ACK, or TCP Retransmissions.
>> That causes packet re-ordering and/or performance issues.
>>
>> TSQ breaks because it orphans the packet while it is still in the
>> host, so packets are queued contributing to the buffer bloat problem.
> 
> Why should TSQ in one stack care about buffer bloat in another stack?
> 
> Actually, I think the current behavior is correct, once the packet leaves
> its current stack (or netns), it should relief the backpressure on TCP
> socket in this stack, whether it will be queued in another stack is beyond
> its concern. This breaks the isolation between networking stacks.
> 

We discussed about this during netconf Cong, nobody was against this planned removal.

When a packet is attached to a socket, we should keep the association as much as possible.

Only when a new association needs to be done, skb_orphan() needs to be called.

Doing this skb_orphan() too soon breaks back pressure in general, this is bad, since a socket
can evades SO_SNDBUF limits.

I am not sure why the patch is so complex, I would have simply removed the skb_orphan().