Message-ID: <52CBC22D.3050002@redhat.com>
Date:	Tue, 07 Jan 2014 17:00:29 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	John Fastabend <john.r.fastabend@...el.com>
CC:	John Fastabend <john.fastabend@...il.com>,
	Neil Horman <nhorman@...driver.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	mst@...hat.com, Vlad Yasevich <vyasevic@...hat.com>
Subject: Re: [PATCH net 1/2] macvlan: forbid L2 fowarding offload for macvtap

On 01/07/2014 03:26 PM, John Fastabend wrote:
> [...]
>
>>>> 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.
>
> Yes.
>
>>
>> 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.

It was introduced in commit 8a35747a5d13b99e076b0222729e0caa48cb69b6
("macvtap: Limit packet queue length") to limit the length of socket
receive queue of macvtap. But I'm not sure whether the qdisc is a
byproduct of this commit, maybe we can switch to use another name
instead of just reuse dev->tx_queue_length.
>
> Also if XPS is not configured then skb_tx_hash is used so multiple
> transmit queues will still be used.
>

True.
>>> 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.
>
> OK.
>
>>> 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. 

At very least the txq of lower device should be held in order to be
synchronized with management path. Consider txq lock were often held by
netif_tx_disable() before trying to down the card. Current cold does not
hold txq lock, so it loses the synchronization which may cause issues.
And the code also does not check whether the txq has been stopped before
trying to start the transmission.


> 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.
>

That will be a nice feature.
>>> 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.

Yes and this is also true if we only do synchronization on the lower
device since only dedicated queues could be selected.
>
> 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
> macvtap.
>

As I said above, holding the txq lock of lower device seems a must and
we should not get regression if NETIF_F_LLTX is kept. But I agree we
need some test.
>>
>>>
>>>
>>>> - 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.
>
> Right.
>
>>>
>>>> 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.
>

It's much safer to disable l2 forwarding offload for macvtap temporarily
consider it has several issues.We can re-enable it when everything is
ready in net-next. We we really need to hold the txq lock of lower
device,  only add more check of NULL pointer is not sufficient. So
explicitly select a txq is still needed. And I don't see any conflicts
between this and future enhancement.

Also I don't see any drawback of using NETIF_F_LLTX for l2 forwarding.
So we'd better keep it.
> 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.
>
>>>
>>> Thanks,
>>> John
>>>
>>
>> Thanks
>>
>
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