I don't think this is going to help __multipath_map() without some
configuration changes. Now that we're running on already merged
requests instead of bios, the m->repeat_count is almost always set to 1,
so we call the path_selector every time, which means that we'll always
need the write lock. Bumping up the number of IOs we send before calling
the path selector again will give this patch a change to do some good
here.
To do that you need to set:
rr_min_io_rq <something_bigger_than_one>
in the defaults section of /etc/multipath.conf and then reload the
multipathd service.
The patch should hopefully help in multipath_busy() regardless of the
the rr_min_io_rq setting.
This patch, while generic, is meant to help the blk-mq case. A blk-mq
request_queue doesn't have an elevator so the requests will not have
seen merging.
But yes, implied in the patch is the requirement to increase
m->repeat_count via multipathd's rr_min_io_rq (I'll backfill a proper
header once it is tested).
I'll test it once I get some spare time (hopefully soon...)
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