Date:	Fri, 17 Jan 2014 13:22:29 +0800
From:	Stefan Hajnoczi <stefanha@...hat.com>
To:	Tom Herbert <therbert@...gle.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 09:12:29AM -0800, Tom Herbert wrote:
> 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.

I guess we need to see benchmark results :)

> > 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.

There are a lot of details that are not yet worked out:

If you want to implement aRFS down the vhost_net + macvtap path
(probably easiest?) how will Step 2 work?  Do the necessary kernel
interfaces exist to take the flow information in vhost_net, give them to
macvtap, and finally push them down to the physical NIC?

Not sure if aRFS will work down the full stack with vhost_net + tap +
bridge.  Any ideas?

At the QEMU level it is currently pointless to implement virtio-net aRFS
emulation since the QEMU global mutex is taken and virtio-net emulation
is not multi-threaded.

I think aRFS is a good thing, we just need to see performance results
and know that this won't be a dead end after merging changes to
virtio-net and the virtio specification.

Stefan
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