Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

[Ming Lei <ming.lei@redhat.com>]

On Mon, Oct 16, 2017 at 01:30:09PM +0200, Hannes Reinecke wrote:
> On 10/13/2017 07:29 PM, Ming Lei wrote:
> > On Fri, Oct 13, 2017 at 05:08:52PM +0000, Bart Van Assche wrote:
> >> On Sat, 2017-10-14 at 00:45 +0800, Ming Lei wrote:
> >>> On Fri, Oct 13, 2017 at 04:31:04PM +0000, Bart Van Assche wrote:
> >>>> On Sat, 2017-10-14 at 00:07 +0800, Ming Lei wrote:
> >>>>> Actually it is in hot path, for example, lpfc and qla2xx's queue depth is 3,
> >>>>
> >>>> Sorry but I doubt whether that is correct. More in general, I don't know any modern
> >>>> storage HBA for which the default queue depth is so low.
> >>>
> >>> You can grep:
> >>>
> >>> [ming@ming linux]$ git grep -n cmd_per_lun ./drivers/scsi/ | grep -E "qla2xxx|lpfc"
> >>
> >> Such a low queue depth will result in suboptimal performance for adapters
> >> that communicate over a storage network. I think that's a bug and that both
> >> adapters support much higher cmd_per_lun values.
> >>
> >> (+James Smart)
> >>
> >> James, can you explain us why commit 445cf4f4d2aa decreased LPFC_CMD_PER_LUN
> >> from 30 to 3? Was that perhaps a workaround for a bug in a specific target
> >> implementation?
> >>
> >> (+Himanshu Madhani)
> >>
> >> Himanshu, do you perhaps know whether it is safe to increase cmd_per_lun for
> >> the qla2xxx initiator driver to the scsi_host->can_queue value?
> > 
> > ->can_queue is size of the whole tag space shared by all LUNs, looks it isn't
> > reasonable to increase cmd_per_lun to .can_queue.
> > 
> '3' is just a starting point; later on it'll be adjusted via
> scsi_change_depth().
> Looks like it's not working correctly with blk-mq, though.

At default, in scsi_alloc_sdev(), q->queue_depth is set as
host->cmd_per_lun. You are right, q->queue_depth can be adjusted
later too.

q->queue_depth is respected in scsi_dev_queue_ready().
.cmd_per_lun defines the max outstanding cmds for each lun, I
guess it is respected by some hardware inside.

For example, I remembered that on lpfc q->queue_depth is 30 because
the default 'lpfc_lun_queue_depth' is 30. And its .cmd_per_lun is 3.
Per my observation, this .cmd_per_lun limit is still workable.

-- 
Ming