net: drop_monitor: make last_rx timestamp private

It was recently pointed out to me that the last_rx field of the
net_device structure wasn't updated regularly.  In fact only the
bonding driver really uses it currently.  Since the drop_monitor code
relies on the last_rx field to detect drops on recevie in hardware, We
need to find a more reliable way to rate limit our drop checks (so
that we don't check for drops on every frame recevied, which would be
inefficient.  This patch makes a last_rx timestamp that is private to
the drop monitor code and is updated for every device that we track.
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/drop_monitor.c

@@ -52,6 +52,7 @@ struct per_cpu_dm_data {
 
 struct dm_hw_stat_delta {
 	struct net_device *dev;
+	unsigned long last_rx;
 	struct list_head list;
 	struct rcu_head rcu;
 	unsigned long last_drop_val;
@@ -180,18 +181,25 @@ static void trace_napi_poll_hit(struct napi_struct *napi)
 	struct dm_hw_stat_delta *new_stat;
 
 	/*
-	 * Ratelimit our check time to dm_hw_check_delta jiffies
+	 * Don't check napi structures with no associated device
 	 */
-	if (!napi->dev ||
-	    !time_after(jiffies, napi->dev->last_rx + dm_hw_check_delta))
+	if (!napi->dev)
 		return;
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(new_stat, &hw_stats_list, list) {
+		/*
+		 * only add a note to our monitor buffer if:
+		 * 1) this is the dev we received on
+		 * 2) its after the last_rx delta
+		 * 3) our rx_dropped count has gone up
+		 */
 		if ((new_stat->dev == napi->dev)  &&
+		    (time_after(jiffies, new_stat->last_rx + dm_hw_check_delta)) &&
 		    (napi->dev->stats.rx_dropped != new_stat->last_drop_val)) {
 			trace_drop_common(NULL, NULL);
 			new_stat->last_drop_val = napi->dev->stats.rx_dropped;
+			new_stat->last_rx = jiffies;
 			break;
 		}
 	}
@@ -287,6 +295,7 @@ static int dropmon_net_event(struct notifier_block *ev_block,
 			goto out;
 
 		new_stat->dev = dev;
+		new_stat->last_rx = jiffies;
 		INIT_RCU_HEAD(&new_stat->rcu);
 		spin_lock(&trace_state_lock);
 		list_add_rcu(&new_stat->list, &hw_stats_list);