From: Flavio Leitner <fbl@sysclose•org>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: netdev@vger•kernel.org, Paolo Abeni <pabeni@redhat•com>
Subject: Re: Poor TCP performance with XPS enabled after scrubbing skb
Date: Thu, 24 May 2018 16:17:29 -0300 [thread overview]
Message-ID: <20180524191729.GA3770@plex.lan> (raw)
In-Reply-To: <c8f6c2f8-c590-2912-c7af-8bce717480b6@gmail.com>
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
next prev parent reply other threads:[~2018-05-24 19:17 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-15 19:31 Poor TCP performance with XPS enabled after scrubbing skb Flavio Leitner
2018-05-15 21:08 ` Eric Dumazet
2018-05-24 19:17 ` Flavio Leitner [this message]
2018-05-25 20:29 ` David Miller
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