netdev - Re: [PATCH net 2/2] net: core: explicitly select a txq before doing l2 forwarding

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Date:	Mon, 6 Jan 2014 10:29:14 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	Jason Wang <jasowang@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	mst@...hat.com, John Fastabend <john.r.fastabend@...el.com>,
	e1000-devel@...ts.sourceforge.net
Subject: Re: [PATCH net 2/2] net: core: explicitly select a txq before doing
 l2 forwarding

On Mon, Jan 06, 2014 at 07:06:25AM -0800, John Fastabend wrote:
> On 01/06/2014 04:42 AM, 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@...el.com>
> >>Cc: Neil Horman <nhorman@...driver.com>
> >>Cc: e1000-devel@...ts.sourceforge.net
> >>Signed-off-by: Jason Wang <jasowang@...hat.com>
> >
> >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.
> >
> >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?
> >
> >Also, can you elaborate on what you mean by additional lock contention?  What
> >contention do you see that goes above and beyond the normal locking required by
> >txq access?  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.
> >
> 
> NETIF_F_LLTX is cleared in macvlan_open() which should be used in the
> macvtap case.
> 
Thats right, since accelerated hardware tx queue doesn't participate in the
network stack queue locking, the upper device needs to do it.

Thanks!
Neil

>           if (lowerdev->features & NETIF_F_HW_L2FW_DOFFLOAD) {
>                   vlan->fwd_priv =
> 
> lowerdev->netdev_ops->ndo_dfwd_add_station(lowerdev, dev);
> 
>                   /* If we get a NULL pointer back, or if we get an error
>                    * then we should just fall through to the non
> accelerated path
>                    */
>                   if (IS_ERR_OR_NULL(vlan->fwd_priv)) {
>                           vlan->fwd_priv = NULL;
>                   } else {
>                           dev->features &= ~NETIF_F_LLTX;
>                           return 0;
>                   }
>           }
> 
> Thanks,
> John
> 
> 
> -- 
> John Fastabend         Intel Corporation
> 
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