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SubjectRe: [PATCH net V2 2/2] net: core: explicitly select a txq before doing l2 forwarding
On Thu, Jan 09, 2014 at 05:37:32PM +0800, Jason Wang wrote:
> Currently, the tx queue were selected implicitly in ndo_dfwd_start_xmit(). The
> will cause several issues:
>
> - NETIF_F_LLTX were removed for macvlan, so txq lock were done for macvlan
> instead of lower device which misses the necessary txq synchronization for
> lower device such as txq stopping or frozen required by dev watchdog or
> control path.
> - 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 new param for .ndo_select_queue() for just
> selecting queues in the case of l2 forwarding offload. And also introducing
> dfwd_direct_xmit() to do the queue selecting, txq holding 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(). Also there's no need to keep
> a dedicated ndo_dfwd_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@intel.com>
> Cc: Neil Horman <nhorman@tuxdriver.com>
> Cc: e1000-devel@lists.sourceforge.net
> Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> ---
> Changes from V1:
> - Adding a new parameter to ndo_select_queue instead of a new method to select
> queue for l2 forwarding.
> - Remove the unnecessary ndo_dfwd_start_xmit() since txq was selected
> explicitly.
> - Keep NETIF_F_LLTX when netdev feature is changed.
> - Shape the commit log

A few minor nits inline.
><snip>

> }
> diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> index 5360f73..7eb4c82 100644
> --- a/drivers/net/macvlan.c
> +++ b/drivers/net/macvlan.c
> @@ -299,7 +299,7 @@ netdev_tx_t macvlan_start_xmit(struct sk_buff *skb,
>
> if (vlan->fwd_priv) {
> skb->dev = vlan->lowerdev;
> - ret = dev_hard_start_xmit(skb, skb->dev, NULL, vlan->fwd_priv);
> + ret = dfwd_direct_xmit(skb, skb->dev, vlan->fwd_priv);
> } else {
> ret = macvlan_queue_xmit(skb, dev);
> }
> @@ -366,7 +366,6 @@ static int macvlan_open(struct net_device *dev)
> if (IS_ERR_OR_NULL(vlan->fwd_priv)) {
> vlan->fwd_priv = NULL;
> } else {
> - dev->features &= ~NETIF_F_LLTX;
> return 0;
> }
After removing the features flag operation here, you don't need the braces
around the else statement either.

><snip>
> +int dfwd_direct_xmit(struct sk_buff *skb, struct net_device *dev,
> + void *accel_priv)
> +{
> + struct netdev_queue *txq;
> + int ret = NETDEV_TX_BUSY;
> + int index;
> +
> + BUG_ON(!dev->netdev_ops->ndo_select_queue);
> + index = dev->netdev_ops->ndo_select_queue(dev, skb, accel_priv);
> +
> + local_bh_disable();
> +
> + skb_set_queue_mapping(skb, index);
> + txq = netdev_get_tx_queue(dev, index);
> +
> + HARD_TX_LOCK(dev, txq, smp_processor_id());
> + if (!netif_xmit_frozen_or_stopped(txq))
> + ret = dev_hard_start_xmit(skb, dev, txq);
> + HARD_TX_UNLOCK(dev, txq);
> +
> + local_bh_enable();
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(dfwd_direct_xmit);
> +
Now that we're using the common path to select a queue, can we just use
dev_queue_xmit here instead of creating our own transmit function? The txq we
select from the ixgbe card will just have a pfifo_fast queue on it (if not a
noop queue), so dev_queue_xmit should just fall into the dev_hard_start_xmit
path, and save us this extra coding.

Neil



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