net-rfs: fix false sharing accessing sd->input_queue_head

sd->input_queue_head is incremented for each processed packet
in process_backlog(), and read from other cpus performing
Out Of Order avoidance in get_rps_cpu()

Moving this field in a separate cache line keeps it mostly
hot for the cpu in process_backlog(), as other cpus will
only read it.

In a stress test, process_backlog() was consuming 6.80 % of cpu cycles,
and the patch reduced the cost to 0.65 %

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2747,11 +2747,15 @@ struct softnet_data {
 	struct sk_buff		*completion_queue;
 
 #ifdef CONFIG_RPS
-	/* Elements below can be accessed between CPUs for RPS */
+	/* input_queue_head should be written by cpu owning this struct,
+	 * and only read by other cpus. Worth using a cache line.
+	 */
+	unsigned int		input_queue_head ____cacheline_aligned_in_smp;
+
+	/* Elements below can be accessed between CPUs for RPS/RFS */
 	struct call_single_data	csd ____cacheline_aligned_in_smp;
 	struct softnet_data	*rps_ipi_next;
 	unsigned int		cpu;
-	unsigned int		input_queue_head;
 	unsigned int		input_queue_tail;
 #endif
 	unsigned int		dropped;