Subject: Re: Poor TCP performance with XPS enabled after scrubbing skb

From: Flavio Leitner <[email-masked]>
Date: Thu, 24 May 2018 16:17:29 -0300

On Tue, May 15, 2018 at 02:08:09PM -0700, Eric Dumazet wrote:
> 
> 
> On 05/15/2018 12:31 PM, Flavio Leitner wrote:
> > Hi,
> > 
> > There is a significant throughput issue (~50% drop) for a single TCP
> > stream when the skb is scrubbed and XPS is enabled.
> > 
> > If I turn CONFIG_XPS off, then the issue never happens and the test
> > reaches line rate.  The same happens if I echo 0 to tx-*/xps_cpus.
> > 
> > It looks like that when the skb is scrubbed, there is no more reference
> > to the struct sock, 
> 
> And this is really the problem here, since it breaks back pressure (and TCP Small queues)
> 
> I am not sure why skb_orphan() is used in this scrubbing really.
> 

veth originally called skb_orphan() on veth_xmit() most probably
because there was no TX completion. Then the code got generalized to
dev_forward_skb() and later on moved to skb_scrub_packet().

The issue is that we call skb_scrub_packet() on TX and RX paths and
that is done while crossing netns.  It doesn't look correct to keep
the ->sk because I suspect that iptables/selinux/bpf, or some code
path that I am probably missing could expose/use the wrong ->sk, for
example.

However, netdev_pick_tx() can't store the queue mapping without ->sk.

The hack in the first email relies on the headers (skb_tx_hash) to
always selected the same TX queue, which solves the original problem
but not the TCP small queues you mentioned.

-- 
Flavio