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.fastab...@intel.com>
> >>Cc: Neil Horman <nhor...@tuxdriver.com>
> >>Cc: e1000-de...@lists.sourceforge.net
> >>Signed-off-by: Jason Wang <jasow...@redhat.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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