dpif-netdev: XPS (Transmit Packet Steering) implementation.

dpif-netdev: XPS (Transmit Packet Steering) implementation.

If CPU number in pmd-cpu-mask is not divisible by the number of queues and
in a few more complex situations there may be unfair distribution of TX
queue-ids between PMD threads.

For example, if we have 2 ports with 4 queues and 6 CPUs in pmd-cpu-mask
such distribution is possible:
<------------------------------------------------------------------------>
pmd thread numa_id 0 core_id 13:
        port: vhost-user1       queue-id: 1
        port: dpdk0     queue-id: 3
pmd thread numa_id 0 core_id 14:
        port: vhost-user1       queue-id: 2
pmd thread numa_id 0 core_id 16:
        port: dpdk0     queue-id: 0
pmd thread numa_id 0 core_id 17:
        port: dpdk0     queue-id: 1
pmd thread numa_id 0 core_id 12:
        port: vhost-user1       queue-id: 0
        port: dpdk0     queue-id: 2
pmd thread numa_id 0 core_id 15:
        port: vhost-user1       queue-id: 3
<------------------------------------------------------------------------>

As we can see above dpdk0 port polled by threads on cores:
	12, 13, 16 and 17.

By design of dpif-netdev, there is only one TX queue-id assigned to each
pmd thread. This queue-id's are sequential similar to core-id's. And
thread will send packets to queue with exact this queue-id regardless
of port.

In previous example:

	pmd thread on core 12 will send packets to tx queue 0
	pmd thread on core 13 will send packets to tx queue 1
	...
	pmd thread on core 17 will send packets to tx queue 5

So, for dpdk0 port after truncating in netdev-dpdk:

	core 12 --> TX queue-id 0 % 4 == 0
	core 13 --> TX queue-id 1 % 4 == 1
	core 16 --> TX queue-id 4 % 4 == 0
	core 17 --> TX queue-id 5 % 4 == 1

As a result only 2 of 4 queues used.

To fix this issue some kind of XPS implemented in following way:

	* TX queue-ids are allocated dynamically.
	* When PMD thread first time tries to send packets to new port
	  it allocates less used TX queue for this port.
	* PMD threads periodically performes revalidation of
	  allocated TX queue-ids. If queue wasn't used in last
	  XPS_TIMEOUT_MS milliseconds it will be freed while revalidation.
        * XPS is not working if we have enough TX queues.

Reported-by: Zhihong Wang <zhihong.wang@intel.com>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>