RFC net:setup XPS mapping for each online CPU

Currently, most MQ netdevice setup the default XPS configuration mapping 1-1 the first real_num_tx_queues queues and CPUs and no mapping is created for the CPUs with id greater then real_num_tx_queues, if any.

As a result, the xmit path for unconnected sockets on such cores experiences a relevant overhead in netdev_pick_tx(), which needs to dissect each packet and compute its hash.

Such scenario is easily triggered. e.g. from DNS server under relevant load, as the user-space process is moved away from the CPUs serving the softirqs (note:
this is beneficial for the overall DNS server performances).

This series introduces an helper to easily setup up XPS mapping for all the online CPUs, and use it in the ixgbe driver, demonstrating a relevant performance improvement in the above scenario.

Paolo Abeni (2):
net: introduce netif_set_xps()
ixgbe: setup XPS via netif_set_xps()

 drivers/net/ethernet/intel/ixgbe/ixgbe.h         |  2 +-
 drivers/net/ethernet/intel/ixgbe/ixgbe_ethtool.c |  1 +
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c    | 13 ++----
 include/linux/netdevice.h                        |  6 +++
 net/core/dev.c                                   | 58 ++++++++++++++++++------
 5 files changed, 55 insertions(+), 25 deletions(-)

2.14.3