Re: [net-next RFC V3 PATCH 4/6] tuntap: multiqueue support

From: Jason Wang
Date: Thu Jun 28 2012 - 01:29:26 EST

On 06/28/2012 12:52 PM, Sridhar Samudrala wrote:
On 6/27/2012 8:02 PM, Jason Wang wrote:
On 06/27/2012 04:44 PM, Michael S. Tsirkin wrote:
On Wed, Jun 27, 2012 at 01:16:30PM +0800, Jason Wang wrote:
On 06/26/2012 06:42 PM, Michael S. Tsirkin wrote:
On Tue, Jun 26, 2012 at 11:42:17AM +0800, Jason Wang wrote:
On 06/25/2012 04:25 PM, Michael S. Tsirkin wrote:
On Mon, Jun 25, 2012 at 02:10:18PM +0800, Jason Wang wrote:
This patch adds multiqueue support for tap device. This is done by abstracting
each queue as a file/socket and allowing multiple sockets to be attached to the
tuntap device (an array of tun_file were stored in the tun_struct). Userspace
could write and read from those files to do the parallel packet
sending/receiving.

Unlike the previous single queue implementation, the socket and device were
loosely coupled, each of them were allowed to go away first. In order to let the
tx path lockless, netif_tx_loch_bh() is replaced by RCU/NETIF_F_LLTX to
synchronize between data path and system call.
Don't use LLTX/RCU. It's not worth it.
Use something like netif_set_real_num_tx_queues.

The tx queue selecting is first based on the recorded rxq index of an skb, it
there's no such one, then choosing based on rx hashing (skb_get_rxhash()).

Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
Interestingly macvtap switched to hashing first:
ef0002b577b52941fb147128f30bd1ecfdd3ff6d
(the commit log is corrupted but see what it
does in the patch).
Any idea why?
Yes, so tap should be changed to behave same as macvtap. I remember
the reason we do that is to make sure the packet of a single flow to
be queued to a fixed socket/virtqueues. As 10g cards like ixgbe
choose the rx queue for a flow based on the last tx queue where the
packets of that flow comes. So if we are using recored rx queue in
macvtap, the queue index of a flow would change as vhost thread
moves amongs processors.
Hmm. OTOH if you override this, if TX is sent from VCPU0, RX might land
on VCPU1 in the guest, which is not good, right?
Yes, but better than making the rx moves between vcpus when we use
recorded rx queue.
Why isn't this a problem with native TCP?
I think what happens is one of the following:
- moving between CPUs is more expensive with tun
because it can queue so much data on xmit
- scheduler makes very bad decisions about VCPUs
bouncing them around all the time

For usual native TCP/host process, as it reads and writes tcp sockets, so it make make sense to move rx to the porcessor where the process moves. But vhost does not do tcp stuffs and ixgbe would still move rx when vhost process moves, and we can't even make sure the vhost process that handling rx is running on processor that handle rx interrupt.

We also saw this behavior with the default ixgbe configuration. If vhost is pinned to a CPU all
packets for that VM are received on a single RX queue.
So even if the VM is doing multiple TCP_RR sessions, packets for all the flows are received
on a single RX queue. Without pinning, vhost moves around and so does the packets across
the RX queues.

I think
ethtool -K ethX ntuple on
will disable this behavior and it should be possible to program the flow director using ethtool -U.
This way we can split the packets across the host NIC RX queues based on the flows, but it is not
clear if this would help with the current model of single vhost per device.
With per-cpu vhost, each RX queue can be handled by the matching vhost, but if we have only
1 queue in the VMs virtio-net device, that could become the bottleneck.

Yes, I've been thinking about this. And instead of using ethtool -U (maybe possible for macvtap but hard for tuntap), we can 'teach' the ixgbe of the rxq it would used for a flow because ixgbe_select_queue() would first select the txq based on the recorded rxq. So if we want the flow using a dedicated rxq say N, we can record N to the rxq in tuntap before we passing the skb to bridge.

Multi-queue virtio-net should help here, but we need the same number of queues in VM's virtio-net
device as the host's NIC so that each vhost can handle the corresponding virtio queue.
But if the VM has only 2 vcpus, i think it is not efficient to have 8 virtio-net queues.(to match a host
with 8 physical cpus and 8 RX queues in the NIC).

Ideally, if we can 2 queues in guest, it's better to only use 2 queues in host to avoid extra contention.

Thanks
Sridhar


Could we isolate which it is? Does the problem
still happen if you pin VCPUs to host cpus?
If not it's the queue depth.

It may not help as tun does not record the vcpu/queue that send the stream, so it can't transmit the packets back the same vcpu/queue.
Flow steering is needed to make sure the tx and
rx on the same vcpu.
That involves IPI between processes, so it might be
very expensive for kvm.

But during test tun/tap, one interesting thing I find is that even
ixgbe has recorded the queue index during rx, it seems be lost when
tap tries to transmit skbs to userspace.
dev_pick_tx does this I think but ndo_select_queue
should be able to get it without trouble.




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