net: net: add a core netdev->tx_dropped counter

Dropping packets in __dev_queue_xmit() when transmit queue
is stopped (NIC TX ring buffer full or BQL limit reached) currently
outputs a syslog message.

It would be better to get a precise count of such events available in
netdevice stats so that monitoring tools can have a clue.

This extends the work done in caf586e5f23ce
("net: add a core netdev->rx_dropped counter")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1311,9 +1311,10 @@ struct net_device {
 	int			iflink;
 
 	struct net_device_stats	stats;
-	atomic_long_t		rx_dropped; /* dropped packets by core network
-					     * Do not use this in drivers.
-					     */
+
+	/* dropped packets by core network, Do not use this in drivers */
+	atomic_long_t		rx_dropped;
+	atomic_long_t		tx_dropped;
 
 #ifdef CONFIG_WIRELESS_EXT
 	/* List of functions to handle Wireless Extensions (instead of ioctl).

net/core/dev.c

@@ -2880,6 +2880,7 @@ static int __dev_queue_xmit(struct sk_buff *skb, void *accel_priv)
 	rc = -ENETDOWN;
 	rcu_read_unlock_bh();
 
+	atomic_long_inc(&dev->tx_dropped);
 	kfree_skb(skb);
 	return rc;
 out:
@@ -6238,6 +6239,7 @@ struct rtnl_link_stats64 *dev_get_stats(struct net_device *dev,
 		netdev_stats_to_stats64(storage, &dev->stats);
 	}
 	storage->rx_dropped += atomic_long_read(&dev->rx_dropped);
+	storage->tx_dropped += atomic_long_read(&dev->tx_dropped);
 	return storage;
 }
 EXPORT_SYMBOL(dev_get_stats);