Date:	Thu, 02 Jun 2011 21:52:33 +0200
From:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org,
	Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bonding: reset queue mapping prior to transmission to
 physical device

Le 02/06/2011 21:46, Neil Horman a écrit :
> On Thu, Jun 02, 2011 at 08:09:49PM +0100, Ben Hutchings wrote:
>> On Thu, 2011-06-02 at 14:56 -0400, Neil Horman wrote:
>>> On Thu, Jun 02, 2011 at 07:35:53PM +0100, Ben Hutchings wrote:
>>>> On Thu, 2011-06-02 at 14:03 -0400, Neil Horman wrote:
>>>>> The bonding driver is multiqueue enabled, in which each queue represents a slave
>>>>> to enable optional steering of output frames to given slaves against the default
>>>>> output policy.  However, it needs to reset the skb->queue_mapping prior to
>>>>> queuing to the physical device or the physical slave (if it is multiqueue) could
>>>>> wind up transmitting on an unintended tx queue (one that was reserved for
>>>>> specific traffic classes for instance)
>> [...]
>>>> So far as I can see, this has no effect, because dev_queue_xmit() always
>>>> sets queue_mapping (in dev_pick_tx()).
>>>>
>>> it resets the queue mapping exactly as you would expect it to.  bonding is a
>>> multiqueue enabled device and selects a potentially non-zero queue based on the
>>> output of bond_select_queue.
>>>
>>>> What is the problem you're seeing?
>>>>
>>> The problem is exctly that.  dev_pick_tx() on the bond device sets the
>>> queue_mapping as per the result of bond_select_queue (the ndo_select_queue
>>> method for the bonding driver).  The implementation there is based on the use of
>>> tc with bonding, so that output slaves can be selected for certain types of
>>> traffic.  But when that mechanism is used, skb->queue_mapping is preserved when
>>> the bonding driver queues the frame to the underlying slave.  This denies the
>>> slave (if its also a multiqueue device) the opportunity to reselect the queue
>>> properly, because of this call path:
>>>
>>> bond_queue_xmit
>>>   dev_queue_xmit(slave_dev)
>>>    dev_pick_tx()
>>>     skb_tx_hash()
>>>      __skb_tx_hash()
>>>
>>> __skb_tx_hash sees that skb_queue_recorded returns true, and assigns a hardware queue mapping
>>> based on what the bonding driver chose using its own internal logic.  Since
>>> bonding uses the multiqueue infrastructure to do slave output selection without
>>> any regard for slave output queue selection, it seems to me we should really
>>> reset the queue mapping to zero so the slave device can pick its own tx queue.
>>
>> So you're effectively clearing the *RX queue* number (as this is before
>> dev_pick_tx()) in order to influence TX queue selection.
>>
> No, you're not seeing it properly.  In bonding (as with all stacked devices) we
> make two passes through dev_pick_tx.
>
> 1) The first time we call dev_pick_tx is when the network stack calls
> dev_queue_xmit on the bond device.  here we call ndo_select_queue, which calls
> bond_select_queue.  This method tells the stack which queue to enqueue the skb
> on for the bond device.  We can use tc's skbedit action to select a particular
> queue on the bond device for various classes of traffic, and those queues
> correspond to individual slave devices.
>
> 2) the second time we call dev_pick_tx is when the bonding bond_queue_xmit
> routine (which is the bonding drivers ndo_start_xmit method, called after the
> frist dev_pick_tx), calls dev_queue_xmit, passing the slave net_device pointer).
> In this case we do whatever the slave device has configured (either reset the
> queue to zero, or call the slaves ndo_select queue method).
>
> What I'm fixing is the fact that the bonding drivers queue_mappnig value is
> 'leaking' down to the slave.
>
> Lets say, for example we're bonding two multiqueue devices, and have the bonding
> driver configured to send all traffic to 192.168.1.1 over the first slave (which
> we can accomplish using an appropriate tc rule on the bond device, setting
> frames with that dest ip to have a skb->queue_mapping of 1).
>
> In the above example, frames bound for 192.168.1.1 have skb->queue_mapping set
> to 1 when they enter bond_queue_xmit.  bond_queue_xmit, calls
> dev_queue_xmit(slave, skb), which calls dev_pick_tx(slave, skb).  If we (for
> simplicity's sake) assume the slave has no ndo_select_queue method, dev_pick_tx
> calls __skb_tx_hash to determine the output queue.  But the bonding driver
> already set skb->queue_mapping to 1 (because it wanted this frame output on the
> first slave, not because it wanted to transmit the frame on the slaves tx queue
> 1).  Nevertheless, __skb_tx_hash() sees that skb_rx_queue_recorded returns true
> here and as a result, calls skb_rx_get_queue, which returns 0.  Because of that
> we wind up transmitting on hardware queue 0 all the time, which is not at all
> what we want.  What we want is for the bonding driver to be able to use
> queue_mapping for its own purposes, and then allow the physical device to make
> its own output queue selection independently.  To do this, queue_mapping needs
> to be zero, prior to queuenig the skb to the slave, which is exactly what this
> patch does.

This sounds good to me.

Reviewed-by: Nicolas de Pesloüan <nicolas.2p.debian@...e.fr>

>
>> Here, the bonding device seems to be behaving as a forwarding device.
>> If TX queue selection can go wrong for certain combinations of queue
>> configuration when forwarding, then this is a problem for IP forwarding
>> and bridging as well, isn't it?
>>
> Potentially, yes.  I only fixed this because I was looking at bonding and its
> queue_mapping behavior, and saw that this needed fixing.  Bridging and IP
> forwarding should also likely clear the queue mapping in the forwarding path
> somewhere to avoid selecting an output tx queue that is a function of whatever
> queue and device it arrived on during ingress.  I've not yet looked to see if
> thats already being done.
>
> Neil
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