netdev - Re: Kernel 4.13.0-rc4-next-20170811 - IP Routing / Forwarding performance vs Core/RSS number / HT on

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Date:   Mon, 14 Aug 2017 18:07:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paweł Staszewski <pstaszewski@...are.pl>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: Kernel 4.13.0-rc4-next-20170811 - IP Routing / Forwarding
 performance vs Core/RSS number / HT on

On Tue, 2017-08-15 at 02:45 +0200, Paweł Staszewski wrote:
> 
> W dniu 2017-08-14 o 18:57, Paolo Abeni pisze:
> > On Mon, 2017-08-14 at 18:19 +0200, Jesper Dangaard Brouer wrote:
> >> The output (extracted below) didn't show who called 'do_raw_spin_lock',
> >> BUT it showed another interesting thing.  The kernel code
> >> __dev_queue_xmit() in might create route dst-cache problem for itself(?),
> >> as it will first call skb_dst_force() and then skb_dst_drop() when the
> >> packet is transmitted on a VLAN.
> >>
> >>   static int __dev_queue_xmit(struct sk_buff *skb, void *accel_priv)
> >>   {
> >>   [...]
> >> 	/* If device/qdisc don't need skb->dst, release it right now while
> >> 	 * its hot in this cpu cache.
> >> 	 */
> >> 	if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
> >> 		skb_dst_drop(skb);
> >> 	else
> >> 		skb_dst_force(skb);
> > I think that the high impact of the above code in this specific test is
> > mostly due to the following:
> >
> > - ingress packets with different RSS rx hash lands on different CPUs
> yes but isn't this normal ?
> everybody that want to ballance load over cores will try tu use as many 
> as possible :)
> With some limitations  ... best are 6 to 7 RSS queues - so need to use 6 
> to 7 cpu cores
> 
> > - but they use the same dst entry, since the destination IPs belong to
> > the same subnet
> typical for ddos - many sources one destination

Nobody hit this issue yet.

We usually change the kernel, given typical workloads.

In this case, we might need per cpu nh_rth_input

Or try to hack the IFF_XMIT_DST_RELEASE flag on the vlan netdev.