net: Allow RX queue selection to seed TX queue hashing.

The idea is that drivers which implement multiqueue RX
pre-seed the SKB by recording the RX queue selected by
the hardware.

If such a seed is found on TX, we'll use that to select
the outgoing TX queue.

This helps get more consistent load balancing on router
and firewall loads.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1904,6 +1904,21 @@ static inline void skb_copy_queue_mapping(struct sk_buff *to, const struct sk_bu
 	to->queue_mapping = from->queue_mapping;
 }
 
+static inline void skb_record_rx_queue(struct sk_buff *skb, u16 rx_queue)
+{
+	skb->queue_mapping = rx_queue + 1;
+}
+
+static inline u16 skb_get_rx_queue(struct sk_buff *skb)
+{
+	return skb->queue_mapping - 1;
+}
+
+static inline bool skb_rx_queue_recorded(struct sk_buff *skb)
+{
+	return (skb->queue_mapping != 0);
+}
+
 #ifdef CONFIG_XFRM
 static inline struct sec_path *skb_sec_path(struct sk_buff *skb)
 {

net/core/dev.c

@@ -1722,6 +1722,13 @@ static u16 simple_tx_hash(struct net_device *dev, struct sk_buff *skb)
 		simple_tx_hashrnd_initialized = 1;
 	}
 
+	if (skb_rx_queue_recorded(skb)) {
+		u32 val = skb_get_rx_queue(skb);
+
+		hash = jhash_1word(val, simple_tx_hashrnd);
+		goto out;
+	}
+
 	switch (skb->protocol) {
 	case htons(ETH_P_IP):
 		if (!(ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)))
@@ -1759,6 +1766,7 @@ static u16 simple_tx_hash(struct net_device *dev, struct sk_buff *skb)
 
 	hash = jhash_3words(addr1, addr2, ports, simple_tx_hashrnd);
 
+out:
 	return (u16) (((u64) hash * dev->real_num_tx_queues) >> 32);
 }