Re: [PATCH net 1/2] macvlan: forbid L2 fowarding offload for macvtap

From: John Fastabend
Date: Tue Jan 07 2014 - 02:27:18 EST


Unfortunately not. This commit has a side effect that it in fact
disables the multiqueue macvtap transmission. Since all macvtap queues
will contend on a single qdisc lock.

They will only contend on a single qdisc lock if the lower device has
1 queue.

I think we are talking about 6acf54f1cf0a6747bac9fea26f34cfc5a9029523.


The qdisc or txq lock were macvlan device itself since dev_queue_xmit()
was called for macvlan device itself. So even if lower device has
multiple txqs, if you just create a one queue macvlan device, you will
get lock contention on macvlan device. And even if you explicitly
specifying the txq numbers ( though I don't believe most management
software will do this) when creating the macvlan/macvtap device, you
must also configure the XPS for macvlan to make sure it has the
possibility of using multiple transmit queues.

OK I think I'm finally putting all the pieces together thanks.

Do you know why macvtap is setting dev->tx_queue_len by default? If you
zero this then the noqueue_qdisc is used and the q->enqueue check in
dev_queue_xmit will fail.

Also if XPS is not configured then skb_tx_hash is used so multiple
transmit queues will still be used.

Perhaps defaulting the L2 forwarding devices to 1queue was a
mistake. But the same issue arises when running macvtap over a
non-multiqueue nic. Or even if you have a multiqueue device and create
many more macvtap queues than the lower device has queues.

Shouldn't the macvtap configuration take into account the lowest level
devices queues?

See commit 8ffab51b3dfc54876f145f15b351c41f3f703195 ("macvlan: lockless
tx path"). It allows the management to create a device without worrying
the underlying device.


How does using the L2 forwarding device change the
contention issues? Without the L2 forwarding LLTX is enabled but the
qdisc lock, etc is still acquired on the device below the macvlan.

That's the point. We need make sure the txq selection and qdisc lock
were done for the lower device not for the macvlan device itself. Then
macvlan can automatically benefit from the multi-queue capable lower
devices. But L2 forwarding needs to contend on the txq lock on macvlan
device itself, which is unnecessary and can complex the management.

If I make the l2 forwarding defaults a bit better then using the L2
forwarding case should not be any more complex. And because the queues
are dedicated to the macvtap device any contention from qdisc lock, etc
comes from the upper device only. Also if I get the bandwidth controls
in we can set the max/min bandwidth per macvtap device this way. That
is future work though.

The ixgbe driver as it is currently written can be configured for up to
4 queues by setting numtxqueues when the device is created. I assume
when creating macvtap queues the user needs to account for the number
of queues supported by the lower device.

We'd better not complicate the task of management, lockless tx path work
very well so we can just keep it. Btw, there's no way for the user to
know the maximum number of queues that L2 forwarding supports.

Good point I'll add an attribute to query it.

For L2 forwarding offload itself, more issues need to be addressed for
multiqueue macvtap:

- ndo_dfwd_add_station() can only create queues per device at ndo_open,
but multiqueue macvtap allows user to create and destroy queues at their
will and at any time.

same argument as above, isn't this the same when running macvtap without
the l2 offloads over a real device? I expect you hit the same contention
points when running over a real device.

Not true and not only for contention.

Macvtap allows user to create or destroy a queue by simply open or close
to character device /dev/tapX. But currently, we do nothing when a new
queue was created or destroyed for L2 forwarding offload.

For contention, lockless tx path make the contention only happens for
the txq or qdisc for the lower device, but L2 forwarding offload make
contention also happen for the macvlan device itself.

Right, but there will be less contention there because those queues
are a dedicated resource for the upper device.

At this point I think I need to put together a real testbed and
benchmark some of this with netperf and perf running to get real
numbers. When I originally did the l2 forwarding I did not do any
testing with multiple macvtap queues and only very limited work with

- it looks that ixgbe has a upper limit of 4 queues per station, but
macvtap currently allows up to 16 queues per device.

The 4 limit was to simplify the code because the queue mapping in the
driver gets complicated if it is greater than 4. We can probably
increase this latter. But sorry reiterating how is this different than
a macvtap on a real device that supports a max of 4 queues?

Well, it maybe easy. I just point out possible issues we may meet currently.


So more works need to be done and unless those above 3 issues were
addressed, this patch is really needed to make sure macvtap works.

Agreed there is a lot more work here to improve things I'm just not
sure we need to disable this now. Also note its the l2 forwarding
should be disabled by default so a user would have to enable the
feature flag.

Even if it was disabled by default. We should not surprise the user who
want to enable it for macvtap.

So the question is what to do in net while we improve net-next. Either
we fix the crash from the null txq and note that with l2 forwarding
some non default configuration is needed for optimal performance OR
for now disable it as your patch does. I would prefer to fix the crash
and note the configuration but I see your point about surprising users
so could go either way.

Neil any thoughts?

To fix the null txq in the gso case adding a check for a non-null
txq before calling txq_trans_update() makes sense to me. We already
have the check in the non-gso case so making it symmetric fixes it.



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