tree 417d31d35bdcda72b55882c8b0137b4799fdb6e8
parent 3bbb198781641cb73aa2934426ca66cad19bd647
author Mike Snitzer <snitzer@redhat.com> 1473463579 -0400
committer Mike Snitzer <snitzer@redhat.com> 1473875798 -0400

dm mpath: delay the requeue of blk-mq requests while all paths down

Return DM_MAPIO_DELAY_REQUEUE from .clone_and_map_rq.  Also, return
false from .busy, if all paths are down, so that blk-mq requests get
mapped via .clone_and_map_rq -- which results in DM_MAPIO_DELAY_REQUEUE
being returned to dm-rq.

This change allows for a noticeable reduction in cpu utilization
(reduced kworker load) while all paths are down, e.g.:

system CPU idleness (as measured by fio's --idle-prof=system):
before: system: 86.58%
after:  system: 98.60%

Signed-off-by: Mike Snitzer <snitzer@redhat.com>