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

[John Fastabend]

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@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.
>
> 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.

          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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