netdev - Re: [PATCH] macvtap: Fix macvtap_get

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Message-ID: <OF12661931.5C4899F8-ON65257952.003B84A6-65257952.003D8DA6@in.ibm.com>
Date:	Thu, 24 Nov 2011 16:44:34 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	arnd@...db.de, davem@...emloft.net, jasowang@...hat.com,
	levinsasha928@...il.com, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first

"Michael S. Tsirkin" <mst@...hat.com> wrote on 11/24/2011 03:29:03 PM:

> Subject Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first
>
> On Thu, Nov 24, 2011 at 01:47:14PM +0530, Krishna Kumar wrote:
> > It was reported that the macvtap device selects a
> > different vhost (when used with multiqueue feature)
> > for incoming packets of a single connection. Use
> > packet hash first. Patch tested on MQ virtio_net.
>
> So this is sure to address the problem, why exactly does this happen?
> Does your device spread a single flow across multiple RX queues? Would
> not that cause trouble in the TCP layer?
> It would seem that using the recorded queue should be faster with
> less cache misses. Before we give up on that, I'd
> like to understand why it's wrong. Do you know?

I am using ixgbe. From what I briefly saw, ixgbe_alloc_rx_buffers
calls skb_record_rx_queue when a skb is allocated. When a packet
is received (ixgbe_alloc_rx_buffers), it sets rxhash. The
recorded value is different for most skbs when I ran a single
stream TCP stream test (does skbs move between the rx_rings?).
With this patch, macvtap selects the same device for each
incoming packet. I can add some debugs in ixgbe to see what is
happening also. I am not sure if Sasha was using a different
device.

Cc'ing Jeffrey in case he can add something.

thanks,

- KK

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