net: add function dev_fetch_sw_netstats for fetching pcpu_sw_netstats

In several places the same code is used to populate rtnl_link_stats64
fields with data from pcpu_sw_netstats. Therefore factor out this code
to a new function dev_fetch_sw_netstats().

v2:
- constify argument netstats
- don't ignore netstats being NULL or an ERRPTR
- switch to EXPORT_SYMBOL_GPL
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/6d16a338-52f5-df69-0020-6bc771a7d498@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -4499,6 +4499,8 @@ struct rtnl_link_stats64 *dev_get_stats(struct net_device *dev,
 					struct rtnl_link_stats64 *storage);
 void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
 			     const struct net_device_stats *netdev_stats);
+void dev_fetch_sw_netstats(struct rtnl_link_stats64 *s,
+			   const struct pcpu_sw_netstats __percpu *netstats);
 
 extern int		netdev_max_backlog;
 extern int		netdev_tstamp_prequeue;

net/core/dev.c

@@ -10328,6 +10328,40 @@ struct rtnl_link_stats64 *dev_get_stats(struct net_device *dev,
 }
 EXPORT_SYMBOL(dev_get_stats);
 
+/**
+ *	dev_fetch_sw_netstats - get per-cpu network device statistics
+ *	@s: place to store stats
+ *	@netstats: per-cpu network stats to read from
+ *
+ *	Read per-cpu network statistics and populate the related fields in @s.
+ */
+void dev_fetch_sw_netstats(struct rtnl_link_stats64 *s,
+			   const struct pcpu_sw_netstats __percpu *netstats)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		const struct pcpu_sw_netstats *stats;
+		struct pcpu_sw_netstats tmp;
+		unsigned int start;
+
+		stats = per_cpu_ptr(netstats, cpu);
+		do {
+			start = u64_stats_fetch_begin_irq(&stats->syncp);
+			tmp.rx_packets = stats->rx_packets;
+			tmp.rx_bytes   = stats->rx_bytes;
+			tmp.tx_packets = stats->tx_packets;
+			tmp.tx_bytes   = stats->tx_bytes;
+		} while (u64_stats_fetch_retry_irq(&stats->syncp, start));
+
+		s->rx_packets += tmp.rx_packets;
+		s->rx_bytes   += tmp.rx_bytes;
+		s->tx_packets += tmp.tx_packets;
+		s->tx_bytes   += tmp.tx_bytes;
+	}
+}
+EXPORT_SYMBOL_GPL(dev_fetch_sw_netstats);
+
 struct netdev_queue *dev_ingress_queue_create(struct net_device *dev)
 {
 	struct netdev_queue *queue = dev_ingress_queue(dev);