netdev - Re: Poor TCP performance with XPS enabled after scrubbing skb

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 24 May 2018 16:17:29 -0300
From:   Flavio Leitner <fbl@...close.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>
Subject: Re: Poor TCP performance with XPS enabled after scrubbing skb

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