[dm-devel] [PATCH v4 6/6] dm rq: Avoid that request processing stalls sporadically

[Bart Van Assche]

On Tue, 2017-04-11 at 12:09 -0400, Mike Snitzer wrote:
> This has no place in dm-mq (or any blk-mq
> driver).  If it is needed it should be elevated to blk-mq core to
> trigger blk_mq_delay_run_hw_queue() when BLK_MQ_RQ_QUEUE_BUSY is
> returned from blk_mq_ops' .queue_rq.

Hello Mike,

If the blk-mq core would have to figure out whether or not a queue is no
longer busy without any cooperation from the blk-mq driver all the blk-mq
core could do is to attempt to rerun that queue from time to time. But at
which intervals should the blk-mq core check whether or not a queue is still
busy? Would it be possible to choose intervals at which to check the queue
state that work well for all block drivers? Consider e.g. at the dm-mpath
driver. multipath_busy() returns true as long as path initialization is in
progress. Path initialization can take a long time. The (indirect) call to
blk_mq_run_queue() from pg_init_done() resumes request processing immediately
after path initialization has finished. Sorry but I don't think it is possible
to invent an algorithm for the blk-mq core that guarantees not only that a
queue is rerun as soon as it is no longer busy but also that avoids that
plenty of CPU cycles are wasted by the blk-mq core for checking whether a
queue is no longer busy.

Bart.