Re: [PATCH v6 2/2] dm: support bio polling

[Jens Axboe <axboe@kernel.dk>]

On 3/7/22 11:53 AM, Mike Snitzer wrote:
> From: Ming Lei <ming.lei@redhat.com>
> 
> Support bio(REQ_POLLED) polling in the following approach:
> 
> 1) only support io polling on normal READ/WRITE, and other abnormal IOs
> still fallback to IRQ mode, so the target io is exactly inside the dm
> io.
> 
> 2) hold one refcnt on io->io_count after submitting this dm bio with
> REQ_POLLED
> 
> 3) support dm native bio splitting, any dm io instance associated with
> current bio will be added into one list which head is bio->bi_private
> which will be recovered before ending this bio
> 
> 4) implement .poll_bio() callback, call bio_poll() on the single target
> bio inside the dm io which is retrieved via bio->bi_bio_drv_data; call
> dm_io_dec_pending() after the target io is done in .poll_bio()
> 
> 5) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
> which is based on Jeffle's previous patch.

It's not the prettiest thing in the world with the overlay on bi_private,
but at least it's nicely documented now.

I would encourage you to actually test this on fast storage, should make
a nice difference. I can run this on a gen2 optane, it's 10x the IOPS
of what it was tested on and should help better highlight where it
makes a difference.

If either of you would like that, then send me a fool proof recipe for
what should be setup so I have a poll capable dm device.

-- 
Jens Axboe