On Wed, 2016-09-28 at 20:54 -0700, Tom Herbert wrote:

xps_flows maintains a per device flow table that is indexed by the
skbuff hash. The table is only consulted when there is no queue saved in
a transmit socket for an skbuff.

Each entry in the flow table contains a queue index and a queue
pointer. The queue pointer is set when a queue is chosen using a
flow table entry. This pointer is set to the head pointer in the
transmit queue (which is maintained by BQL).

The new function get_xps_flows_index that looks up flows in the
xps_flows table. The entry returned gives the last queue a matching flow
used. The returned queue is compared against the normal XPS queue. If
they are different, then we only switch if the tail pointer in the TX
queue has advanced past the pointer saved in the entry. In this
way OOO should be avoided when XPS wants to use a different queue.

There is something I do not understand with this.

If this OOO avoidance is tied to BQL, it means that packets sitting in a
qdisc wont be part of the detection.

So packets of flow X could possibly be queued on multiple qdiscs.