net: fix XPS static_key accounting

Commit 04157469 ("net: Use static_key for XPS maps") introduced a
static key for XPS, but the increments/decrements don't match.

First, the static key's counter is incremented once for each queue, but
only decremented once for a whole batch of queues, leading to large
unbalances.

Second, the xps_rxqs_needed key is decremented whenever we reset a batch
of queues, whether they had any rxqs mapping or not, so that if we setup
cpu-XPS on em1 and RXQS-XPS on em2, resetting the queues on em1 would
decrement the xps_rxqs_needed key.

This reworks the accounting scheme so that the xps_needed key is
incremented only once for each type of XPS for all the queues on a
device, and the xps_rxqs_needed key is incremented only once for all
queues. This is sufficient to let us retrieve queues via
get_xps_queue().

This patch introduces a new reset_xps_maps(), which reinitializes and
frees the appropriate map (xps_rxqs_map or xps_cpus_map), and drops a
reference to the needed keys:
 - both xps_needed and xps_rxqs_needed, in case of rxqs maps,
 - only xps_needed, in case of CPU maps.

Now, we also need to call reset_xps_maps() at the end of
__netif_set_xps_queue() when there's no active map left, for example
when writing '00000000,00000000' to all queues' xps_rxqs setting.

Fixes: 04157469 ("net: Use static_key for XPS maps")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2175,6 +2175,20 @@ static bool remove_xps_queue_cpu(struct net_device *dev,
 	return active;
 }
 
+static void reset_xps_maps(struct net_device *dev,
+			   struct xps_dev_maps *dev_maps,
+			   bool is_rxqs_map)
+{
+	if (is_rxqs_map) {
+		static_key_slow_dec_cpuslocked(&xps_rxqs_needed);
+		RCU_INIT_POINTER(dev->xps_rxqs_map, NULL);
+	} else {
+		RCU_INIT_POINTER(dev->xps_cpus_map, NULL);
+	}
+	static_key_slow_dec_cpuslocked(&xps_needed);
+	kfree_rcu(dev_maps, rcu);
+}
+
 static void clean_xps_maps(struct net_device *dev, const unsigned long *mask,
 			   struct xps_dev_maps *dev_maps, unsigned int nr_ids,
 			   u16 offset, u16 count, bool is_rxqs_map)
@@ -2186,13 +2200,8 @@ static void clean_xps_maps(struct net_device *dev, const unsigned long *mask,
 	     j < nr_ids;)
 		active |= remove_xps_queue_cpu(dev, dev_maps, j, offset,
 					       count);
-	if (!active) {
-		if (is_rxqs_map)
-			RCU_INIT_POINTER(dev->xps_rxqs_map, NULL);
-		else
-			RCU_INIT_POINTER(dev->xps_cpus_map, NULL);
-		kfree_rcu(dev_maps, rcu);
-	}
+	if (!active)
+		reset_xps_maps(dev, dev_maps, is_rxqs_map);
 
 	if (!is_rxqs_map) {
 		for (i = offset + (count - 1); count--; i--) {
@@ -2236,10 +2245,6 @@ static void netif_reset_xps_queues(struct net_device *dev, u16 offset,
 		       false);
 
 out_no_maps:
-	if (static_key_enabled(&xps_rxqs_needed))
-		static_key_slow_dec_cpuslocked(&xps_rxqs_needed);
-
-	static_key_slow_dec_cpuslocked(&xps_needed);
 	mutex_unlock(&xps_map_mutex);
 	cpus_read_unlock();
 }
@@ -2357,9 +2362,12 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 	if (!new_dev_maps)
 		goto out_no_new_maps;
 
+	if (!dev_maps) {
+		/* Increment static keys at most once per type */
 		static_key_slow_inc_cpuslocked(&xps_needed);
 		if (is_rxqs_map)
 			static_key_slow_inc_cpuslocked(&xps_rxqs_needed);
+	}
 
 	for (j = -1; j = netif_attrmask_next(j, possible_mask, nr_ids),
 	     j < nr_ids;) {
@@ -2457,13 +2465,8 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 	}
 
 	/* free map if not active */
-	if (!active) {
-		if (is_rxqs_map)
-			RCU_INIT_POINTER(dev->xps_rxqs_map, NULL);
-		else
-			RCU_INIT_POINTER(dev->xps_cpus_map, NULL);
-		kfree_rcu(dev_maps, rcu);
-	}
+	if (!active)
+		reset_xps_maps(dev, dev_maps, is_rxqs_map);
 
 out_no_maps:
 	mutex_unlock(&xps_map_mutex);