[NET]: Eliminate netif_rx massive packet drops.

Eliminate the throttling behaviour when the netif receive queue fills
because it behaves badly when using high speed networks under load.
The throttling cause multiple packet drops that cause TCP to go into
slow start mode. The same effective patch has been part of BIC TCP and
H-TCP as well as part of Web100.

The existing code drops 100's of packets when the queue fills;
this changes it to individual packet drop-tail.

Signed-off-by: Stephen Hemmminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netdevice.h

···
 	unsigned total;
 	unsigned dropped;
 	unsigned time_squeeze;
-	unsigned throttled;
 	unsigned cpu_collision;
 };
 
···
 
 struct softnet_data
 {
-	int			throttle;
+	struct net_device	*output_queue;
 	struct sk_buff_head	input_pkt_queue;
 	struct list_head	poll_list;
-	struct net_device	*output_queue;
 	struct sk_buff		*completion_queue;
 
 	struct net_device	backlog_dev;	/* Sorry. 8) */

net/core/dev.c

···
  *	Device drivers call our routines to queue packets here. We empty the
  *	queue in the local softnet handler.
  */
-DEFINE_PER_CPU(struct softnet_data, softnet_data) = { 0, };
+DEFINE_PER_CPU(struct softnet_data, softnet_data) = { NULL };
 
 #ifdef CONFIG_SYSFS
 extern int netdev_sysfs_init(void);
···
 
 int netif_rx(struct sk_buff *skb)
 {
-	int this_cpu;
 	struct softnet_data *queue;
 	unsigned long flags;
 
···
 	 * short when CPU is congested, but is still operating.
 	 */
 	local_irq_save(flags);
-	this_cpu = smp_processor_id();
 	queue = &__get_cpu_var(softnet_data);
 
 	__get_cpu_var(netdev_rx_stat).total++;
 	if (queue->input_pkt_queue.qlen <= netdev_max_backlog) {
 		if (queue->input_pkt_queue.qlen) {
-			if (queue->throttle)
-				goto drop;
-
 enqueue:
 			dev_hold(skb->dev);
 			__skb_queue_tail(&queue->input_pkt_queue, skb);
···
 			return NET_RX_SUCCESS;
 		}
 
-		if (queue->throttle)
-			queue->throttle = 0;
-
 		netif_rx_schedule(&queue->backlog_dev);
 		goto enqueue;
 	}
 
-	if (!queue->throttle) {
-		queue->throttle = 1;
-		__get_cpu_var(netdev_rx_stat).throttled++;
-	}
-
-drop:
 	__get_cpu_var(netdev_rx_stat).dropped++;
 	local_irq_restore(flags);
 
···
 	smp_mb__before_clear_bit();
 	netif_poll_enable(backlog_dev);
 
-	if (queue->throttle)
-		queue->throttle = 0;
 	local_irq_enable();
 	return 0;
 }
···
 	struct netif_rx_stats *s = v;
 
 	seq_printf(seq, "%08x %08x %08x %08x %08x %08x %08x %08x %08x\n",
-		   s->total, s->dropped, s->time_squeeze, s->throttled,
+		   s->total, s->dropped, s->time_squeeze, 0,
 		   0, 0, 0, 0, /* was fastroute */
 		   s->cpu_collision );
 	return 0;
···
 
 		queue = &per_cpu(softnet_data, i);
 		skb_queue_head_init(&queue->input_pkt_queue);
-		queue->throttle = 0;
 		queue->completion_queue = NULL;
 		INIT_LIST_HEAD(&queue->poll_list);
 		set_bit(__LINK_STATE_START, &queue->backlog_dev.state);