[PKT_SCHED]: Check subqueue status before calling hard_start_xmit

The only qdiscs that check subqueue state before dequeue'ing are PRIO
and RR.  The other qdiscs, including the default pfifo_fast qdisc,
will allow traffic bound for subqueue 0 through to hard_start_xmit.
The check for netif_queue_stopped() is done above in pkt_sched.h, so
it is unnecessary for qdisc_restart().  However, if the underlying
driver is multiqueue capable, and only sets queue states on subqueues,
this will allow packets to enter the driver when it's currently unable
to process packets, resulting in expensive requeues and driver
entries.  This patch re-adds the check for the subqueue status before
calling hard_start_xmit, so we can try and avoid the driver entry when
the queues are stopped.
Signed-off-by: NPeter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -134,7 +134,7 @@ static inline int qdisc_restart(struct net_device *dev)
 {
 	struct Qdisc *q = dev->qdisc;
 	struct sk_buff *skb;
-	int ret;
+	int ret = NETDEV_TX_BUSY;
 
 	/* Dequeue packet */
 	if (unlikely((skb = dev_dequeue_skb(dev, q)) == NULL))
@@ -145,7 +145,8 @@ static inline int qdisc_restart(struct net_device *dev)
 	spin_unlock(&dev->queue_lock);
 
 	HARD_TX_LOCK(dev, smp_processor_id());
-	ret = dev_hard_start_xmit(skb, dev);
+	if (!netif_subqueue_stopped(dev, skb))
+		ret = dev_hard_start_xmit(skb, dev);
 	HARD_TX_UNLOCK(dev);
 
 	spin_lock(&dev->queue_lock);