rps: Inspect GRE encapsulated packets to get flow hash

Crack open GRE packets in __skb_get_rxhash to compute 4-tuple hash on
in encapsulated packet.  Note that this is used only when the
__skb_get_rxhash is taken, in particular only when the device does
not compute provide the rxhash (ie. feature is disabled).

This was tested by creating a single GRE tunnel between two 16 core
AMD machines.  200 netperf TCP_RR streams were ran with 1 byte
request and response size.

Without patch: 157497 tps, 50/90/99% latencies 1250/1292/1364 usecs
With patch: 325896 tps, 50/90/99% latencies 603/848/1169
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2570,6 +2570,28 @@ void __skb_get_rxhash(struct sk_buff *skb)
 	}
 
 	switch (ip_proto) {
+	case IPPROTO_GRE:
+		if (pskb_may_pull(skb, nhoff + 16)) {
+			u8 *h = skb->data + nhoff;
+			__be16 flags = *(__be16 *)h;
+
+			/*
+			 * Only look inside GRE if version zero and no
+			 * routing
+			 */
+			if (!(flags & (GRE_VERSION|GRE_ROUTING))) {
+				proto = *(__be16 *)(h + 2);
+				nhoff += 4;
+				if (flags & GRE_CSUM)
+					nhoff += 4;
+				if (flags & GRE_KEY)
+					nhoff += 4;
+				if (flags & GRE_SEQ)
+					nhoff += 4;
+				goto again;
+			}
+		}
+		break;
 	default:
 		break;
 	}