net: add cpu_relax to busy poll loop

Add a cpu_relaxt to sk_busy_loop.

Julie Cummings reported performance issues when hyperthreading is on.
Arjan van de Ven observed that we should have a cpu_relax() in the
busy poll loop.

Reported-by: Julie Cummings <julie.a.cummings@intel.com>
Signed-off-by: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/busy_poll.h

@@ -123,6 +123,7 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
 			/* local bh are disabled so it is ok to use _BH */
 			NET_ADD_STATS_BH(sock_net(sk),
 					 LINUX_MIB_BUSYPOLLRXPACKETS, rc);
+		cpu_relax();
 
 	} while (!nonblock && skb_queue_empty(&sk->sk_receive_queue) &&
 		 !need_resched() && !busy_loop_timeout(end_time));