Date:	Thu, 16 Jan 2014 09:12:29 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Stefan Hajnoczi <stefanha@...hat.com>
Cc:	Zhi Yong Wu <zwu.kernel@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jason Wang <jasowang@...hat.com>
Subject: Re: Fwd: [RFC PATCH net-next 0/3] virtio_net: add aRFS support

On Thu, Jan 16, 2014 at 12:52 AM, Stefan Hajnoczi <stefanha@...hat.com> wrote:
> On Thu, Jan 16, 2014 at 04:34:10PM +0800, Zhi Yong Wu wrote:
>> CC: stefanha, MST, Rusty Russel
>>
>> ---------- Forwarded message ----------
>> From: Jason Wang <jasowang@...hat.com>
>> Date: Thu, Jan 16, 2014 at 12:23 PM
>> Subject: Re: [RFC PATCH net-next 0/3] virtio_net: add aRFS support
>> To: Zhi Yong Wu <zwu.kernel@...il.com>
>> Cc: netdev@...r.kernel.org, therbert@...gle.com, edumazet@...gle.com,
>> davem@...emloft.net, Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
>>
>>
>> On 01/15/2014 10:20 PM, Zhi Yong Wu wrote:
>> >
>> > From: Zhi Yong Wu<wuzhy@...ux.vnet.ibm.com>
>> >
>> > HI, folks
>> >
>> > The patchset is trying to integrate aRFS support to virtio_net. In this case,
>> > aRFS will be used to select the RX queue. To make sure that it's going ahead
>> > in the correct direction, although it is still one RFC and isn't tested, it's
>> > post out ASAP. Any comment are appreciated, thanks.
>> >
>> > If anyone is interested in playing with it, you can get this patchset from my
>> > dev git on github:
>> >    git://github.com/wuzhy/kernel.git virtnet_rfs
>> >
>> > Zhi Yong Wu (3):
>> >    virtio_pci: Introduce one new config api vp_get_vq_irq()
>> >    virtio_net: Introduce one dummy function virtnet_filter_rfs()
>> >    virtio-net: Add accelerated RFS support
>> >
>> >   drivers/net/virtio_net.c      |   67 ++++++++++++++++++++++++++++++++++++++++-
>> >   drivers/virtio/virtio_pci.c   |   11 +++++++
>> >   include/linux/virtio_config.h |   12 +++++++
>> >   3 files changed, 89 insertions(+), 1 deletions(-)
>> >
>>
>> Please run get_maintainter.pl before sending the patch. You'd better
>> at least cc virtio maintainer/list for this.
>>
>> The core aRFS method is a noop in this RFC which make this series no
>> much sense to discuss. You should at least mention the big picture
>> here in the cover letter. I suggest you should post a RFC which can
>> run and has expected result or you can just raise a thread for the
>> design discussion.
>>
>> And this method has been discussed before, you can search "[net-next
>> RFC PATCH 5/5] virtio-net: flow director support" in netdev archive
>> for a very old prototype implemented by me. It can work and looks like
>> most of this RFC have already done there.
>>
>> A basic question is whether or not we need this, not all the mq cards
>> use aRFS (see ixgbe ATR). And whether or not it can bring extra
>> overheads? For virtio, we want to reduce the vmexits as much as
>> possible but this aRFS seems introduce a lot of more of this. Making a
>> complex interfaces just for an virtual device may not be good, simple
>> method may works for most of the cases.
>>
>> We really should consider to offload this to real nic. VMDq and L2
>> forwarding offload may help in this case.
>
Adding flow director support would be a good step, Zhi's patches for
support in tun have been merged, so support in virtio-net would be a
good follow on. But, flow-director does have some limitations and
performance issues of it's own (forced pairing between TX and RX
queues, lookup on every TX packet). In the case of virtualization,
aRFS, RSS, ntuple filtering, LRO, etc. can be implemented as software
emulations and so far seems to be wins in most cases. Extending these
down into the stack so that they can leverage HW mechanisms is a good
goal for best performance. It's probably generally true that most of
the offloads commonly available for NICs we'll want in virtualization
path. Of course, we need to deomonstrate that they provide real
performance benefit in this use case.

I believe tying in aRFS (or flow director) into a real aRFS is just a
matter of programming the RFS table properly. This is not the complex
side of the interface, I believe this already works with the tun
patches.

> Zhi Yong and I had an IRC chat.  I wanted to post my questions on the
> list - it's still the same concern I had in the old email thread that
> Jason mentioned.
>
> In order for virtio-net aRFS to make sense there needs to be an overall
> plan for pushing flow mapping information down to the physical NIC.
> That's the only way to actually achieve the benefit of steering:
> processing the packet on the CPU where the application is running.
>
I don't think this is necessarily true. Per flow steering amongst
virtual queues should be beneficial in itself. virtio-net can leverage
RFS or aRFS where it's available.

> If it's not possible or too hard to implement aRFS down the entire
> stack, we won't be able to process the packet on the right CPU.
> Then we might as well not bother with aRFS and just distribute uniformly
> across the rx virtqueues.
>
> Please post an outline of how rx packets will be steered up the stack so
> we can discuss whether aRFS can bring any benefit.
>
1. The aRFS interface for the guest to specify which virtual queue to
receive a packet on is fairly straight forward.
2. To hook into RFS, we need to match the virtual queue to the real
CPU it will processed on, and then program the RFS table for that flow
and CPU.
3. NIC aRFS keys off the RFS tables so it can program the HW with the
correct queue for the CPU.

> Stefan
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