Re: [PATCH 0/7] dm: add full blk-mq support to request-based DM

[Jens Axboe]

On 03/11/2015 09:56 PM, Mike Snitzer wrote:
> This patchset updates request-based DM (used exclussively by DM
> multipath) to support blk-mq I/O path or the old ->request_fn I/O
> path -- the default is still the old ->request_fn I/O path.
>
> Like was done for SCSI, I've exposed a dm-mod.ko 'use_blk_mq' module
> parameter and a CONFIG_DM_MQ_DEFAULT.  I've queued these patches in
> linux-next via linux-dm.git's 'for-next' branch -- my goal is for
> these changes to land upstream in 4.1.
>
> I've tested all 4 supported permutations of rq-based queue stacking:
> 1) blk-mq mpath stacked on blk-mq device(s)
> 2) blk-mq mpath stacked on ->request_fn device(s)
> 3) old ->request_fn mpath stacked on blk-mq device(s)
> 4) old ->request_fn mpath stacked on old ->request_fn device(s)
>
> There is definitely much more room for optimizing the blk-mq mpath
> queue (via dm_mq_ops setup) to have more awareness about the
> characteristics of the underlying blk-mq device(s) -- Jens offered
> some suggestions that are worth pursuing.  Goal being to leverage the
> ability to have multiple software submission queues that map to the
> underlying paths' HW queue.  I expect Keith to be the most likely
> person to pursue these follow-on optimizations given his immediate
> access to NVMe devices, etc.
>
> blk-mq allows for the removal of 2 of the 3 mempools and associated
> allocations that were traditionally performed using old ->request_fn
> I/O path.  The md->bs bioset still remains for cloning a request's
> bios.
>
> I'll be pursuing the possibility of removing the bio cloning that
> happens when cloning a request (it was put in place for partial
> completion of a request's bios but the prospect of avoiding those bio
> clones warrants further review of how useful those partial completions
> are, see: http://www.redhat.com/archives/dm-devel/2014-June/msg00030.html)
>
> Jens and Keith, special thanks for your help answering more blk-mq
> questions than I'd have hoped would be needed while at LSF/MM.
>
> Keith Busch (1):
>    blk-mq: don't wait in blk_mq_queue_enter() if __GFP_WAIT isn't set
>
> Mike Snitzer (6):
>    blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path
>    blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk
>    blk-mq: export blk_mq_run_hw_queues
>    dm: add full blk-mq support to request-based DM
>    dm: optimize dm_mq_queue_rq to _not_ use kthread if using pure blk-mq
>    dm: add 'use_blk_mq' module param and expose in per-device ro sysfs attr

Applied patch 1 to current tree, marked for stable. 2-4 are applied for 
4.1, in for-4.1/core.

--
Jens Axboe

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