Re: vhost + multiqueue + RSS question.

[Jason Wang <jasowang@redhat.com>]

On 11/18/2014 07:05 PM, Michael S. Tsirkin wrote:
> On Tue, Nov 18, 2014 at 11:37:03AM +0800, Jason Wang wrote:
>> > On 11/17/2014 07:58 PM, Michael S. Tsirkin wrote:
>>> > > On Mon, Nov 17, 2014 at 01:22:07PM +0200, Gleb Natapov wrote:
>>>>> > >> > On Mon, Nov 17, 2014 at 12:38:16PM +0200, Michael S. Tsirkin wrote:
>>>>>>> > >>> > > On Mon, Nov 17, 2014 at 09:44:23AM +0200, Gleb Natapov wrote:
>>>>>>>>> > >>>> > > > On Sun, Nov 16, 2014 at 08:56:04PM +0200, Michael S. Tsirkin wrote:
>>>>>>>>>>> > >>>>> > > > > On Sun, Nov 16, 2014 at 06:18:18PM +0200, Gleb Natapov wrote:
>>>>>>>>>>>>> > >>>>>> > > > > > Hi Michael,
>>>>>>>>>>>>> > >>>>>> > > > > > 
>>>>>>>>>>>>> > >>>>>> > > > > >  I am playing with vhost multiqueue capability and have a question about
>>>>>>>>>>>>> > >>>>>> > > > > > vhost multiqueue and RSS (receive side steering). My setup has Mellanox
>>>>>>>>>>>>> > >>>>>> > > > > > ConnectX-3 NIC which supports multiqueue and RSS. Network related
>>>>>>>>>>>>> > >>>>>> > > > > > parameters for qemu are:
>>>>>>>>>>>>> > >>>>>> > > > > > 
>>>>>>>>>>>>> > >>>>>> > > > > >    -netdev tap,id=hn0,script=qemu-ifup.sh,vhost=on,queues=4
>>>>>>>>>>>>> > >>>>>> > > > > >    -device virtio-net-pci,netdev=hn0,id=nic1,mq=on,vectors=10
>>>>>>>>>>>>> > >>>>>> > > > > > 
>>>>>>>>>>>>> > >>>>>> > > > > > In a guest I ran "ethtool -L eth0 combined 4" to enable multiqueue.
>>>>>>>>>>>>> > >>>>>> > > > > > 
>>>>>>>>>>>>> > >>>>>> > > > > > I am running one tcp stream into the guest using iperf. Since there is
>>>>>>>>>>>>> > >>>>>> > > > > > only one tcp stream I expect it to be handled by one queue only but
>>>>>>>>>>>>> > >>>>>> > > > > > this seams to be not the case. ethtool -S on a host shows that the
>>>>>>>>>>>>> > >>>>>> > > > > > stream is handled by one queue in the NIC, just like I would expect,
>>>>>>>>>>>>> > >>>>>> > > > > > but in a guest all 4 virtio-input interrupt are incremented. Am I
>>>>>>>>>>>>> > >>>>>> > > > > > missing any configuration?
>>>>>>>>>>> > >>>>> > > > > 
>>>>>>>>>>> > >>>>> > > > > I don't see anything obviously wrong with what you describe.
>>>>>>>>>>> > >>>>> > > > > Maybe, somehow, same irqfd got bound to multiple MSI vectors?
>>>>>>>>> > >>>> > > > It does not look like this is what is happening judging by the way
>>>>>>>>> > >>>> > > > interrupts are distributed between queues. They are not distributed
>>>>>>>>> > >>>> > > > uniformly and often I see one queue gets most interrupt and others get
>>>>>>>>> > >>>> > > > much less and then it changes.
>>>>>>> > >>> > > 
>>>>>>> > >>> > > Weird. It would happen if you transmitted from multiple CPUs.
>>>>>>> > >>> > > You did pin iperf to a single CPU within guest, did you not?
>>>>>>> > >>> > > 
>>>>> > >> > No, I didn't because I didn't expect it to matter for input interrupts.
>>>>> > >> > When I run iperf on a host rx queue that receives all packets depends
>>>>> > >> > only on a connection itself, not on a cpu iperf is running on (I tested
>>>>> > >> > that).
>>> > > This really depends on the type of networking card you have
>>> > > on the host, and how it's configured.
>>> > >
>>> > > I think you will get something more closely resembling this
>>> > > behaviour if you enable RFS in host.
>>> > >
>>>>> > >> > When I pin iperf in a guest I do indeed see that all interrupts
>>>>> > >> > are arriving to the same irq vector. Is a number after virtio-input
>>>>> > >> > in /proc/interrupt any indication of a queue a packet arrived to (on
>>>>> > >> > a host I can use ethtool -S to check what queue receives packets, but
>>>>> > >> > unfortunately this does not work for virtio nic in a guest)?
>>> > > I think it is.
>>> > >
>>>>> > >> > Because if
>>>>> > >> > it is the way RSS works in virtio is not how it works on a host and not
>>>>> > >> > what I would expect after reading about RSS. The queue a packets arrives
>>>>> > >> > to should be calculated by hashing fields from a packet header only.
>>> > > Yes, what virtio has is not RSS - it's an accelerated RFS really.
>> > 
>> > Strictly speaking, not aRFS. aRFS requires a programmable filter and
>> > needs driver to fill the filter on demand. For virtio-net, this is done
>> > automatically in host side (tun/tap). There's no guest involvement.
> Well guest affects the filter by sending tx packets.
>


Yes, it is.