Re: [PATCH net 2/2] net: core: explicitly select a txq before doingl2 forwarding

[Jason Wang]

On 01/06/2014 08:42 PM, Neil Horman wrote:
> On Mon, Jan 06, 2014 at 11:21:07AM +0800, Jason Wang wrote:
>> Currently, the tx queue were selected implicitly in ndo_dfwd_start_xmit(). The
>> will cause several issues:
>>
>> - NETIF_F_LLTX was forced for macvlan device in this case which lead extra lock
>>   contention.
>> - dev_hard_start_xmit() was called with NULL txq which bypasses the net device
>>   watchdog
>> - dev_hard_start_xmit() does not check txq everywhere which will lead a crash
>>   when tso is disabled for lower device.
>>
>> Fix this by explicitly introducing a select queue method just for l2 forwarding
>> offload (ndo_dfwd_select_queue), and introducing dfwd_direct_xmit() to do the
>> queue selecting and transmitting for l2 forwarding.
>>
>> With this fixes, NETIF_F_LLTX could be preserved for macvlan and there's no need
>> to check txq against NULL in dev_hard_start_xmit().
>>
>> In the future, it was also required for macvtap l2 forwarding support since it
>> provides a necessary synchronization method.
>>
>> Cc: John Fastabend <john.r.fastabend@xxxxxxxxx>
>> Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
>> Cc: e1000-devel@xxxxxxxxxxxxxxxxxxxxx
>> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> Instead of creating another operation here to do special queue selection, why
> not just have ndo_dfwd_start_xmit include a pointer to a pointer in its argument
> list, so it can pass the txq it used back to the caller (dev_hard_start_xmit)?
> ndo_dfwd_start_xmit already knows which queue set to pick from (since their
> reserved for the device doing the transmitting).  It seems more clear to me than
> creating a new netdevice operation.  

See commit 8ffab51b3dfc54876f145f15b351c41f3f703195 ("macvlan: lockless
tx path"). The point is keep the tx path lockless to be efficient and
simplicity for management. And macvtap multiqueue was also implemented
with this assumption. The real contention should be done in the txq of
lower device instead of macvlan itself. This is also needed for
multiqueue macvtap.
>
> As for the crash issue, I'm not sure what you mean.  Where in
> dev_hard_start_xmit would we need to check txq that we're not currently, and
> what crash results?

Well, see current dev_hard_start_xmit(), if lower device does not
support tso or tso is disabled, in gso path:

gso:
...
        txq_trans_update(txq);
        if (unlikely(netif_xmit_stopped(txq) && skb->next))

There's an obvious NULL pointer dereference.
>
> Also, can you elaborate on what you mean by additional lock contention? 

If the lower device has NETIF_F_LLTX, then both macvlan txq lock and the
lock of device itself must be held before doing transmission. In the
case, the macvlan txq lock contention is obvious unnecessary.
>  What
> contention do you see that goes above and beyond the normal locking required by
> txq access?  

As I said above, the point is keeping the lockess tx path and make the
contention of txq for real device instead of macvlan itself.
> I suppose its extra locking above and beyond in the macvtap case,
> where you would otherwise never hit hardware, but that not the only use case,
> and I think the solution there is likely to add some code in the macvlan feature
> set handler so that NETIF_F_LLTX is cleared if you disable the hardware
> forwarding acceleration via ethtool.
>
> Regards
> Neil
>

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