Re: [PATCH RFC] hisi_sas_v3: multiqueue support

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On Mon, Jun 03, 2019 at 11:07:11AM +0100, John Garry wrote:
> On 03/06/2019 10:54, Ming Lei wrote:
> > On Mon, Jun 03, 2019 at 09:57:26AM +0100, John Garry wrote:
> > > > > > Otherwise duplicated slot can be used from different blk-mq hw queue.
> > > > > > 
> > > > > > > 
> > > > > > > > The worsen thing is that V3's actual max queue depth is (4096 - 96), but
> > > > > > > > this patch claims that the device can support (4096 - 96) * 32 command
> > > > > > > > slots
> > > > > 
> > > > > To be clear about the hw, the hw supports max 4096 command tags and has 16
> > > > 
> > > > Is 4096 the max allowed host-wide command tags? Or per-queue's max commands
> > > > tags?
> > > 
> > > 4096 is max allowed host wide, in range [0, 4096), and tags are not per
> > > queue. HW delivery queues can be configured to be as large as desired.
> > 
> > Then all delivery(and complete) queues share the 4096 command slots, we can't
> > convert hisi_sas V3 into typical blk-mq MQ model simply as done by Hannes's patch.
> > 
> > Or you may partition the 4096 tags into 16 queues, then each queue's
> > depth is ~256. Depends on if performance is good for workloads.
> > You still can build a unique tag easily in [0, 4096), such as
> > (req->tag * hw_queue_index).
> > 
> > blk-mq's hw queue has independent tags, which is from NVMe's submission/completion
> > queue.
> 
> ehhhh, and this is what I thought you were addressing in "[PATCH 1/9]
> blk-mq: allow hw queues to share hostwide tags", right?

That patch allows drivers, such as hisi_sas v3, to share the same single
tags among all hw queues, then the private hw queue(reply queue, or
delivery/complete queue) can be converted to blk-mq hw queue.

> 
> > 
> > > 
> > > And on another point I saw mentioned, the device supports multiple
> > > submission and multiple completion queues. They are symmetrical, and any
> > > command will always complete on the same queue index which it was submitted.
> > 
> > DQ & CQ did confuse me a bit, :-)
> 
> DQ is "delivery" queue. Interna; terminolgy.
> 
> In response to earlier "It depends on if hisi_sas V3 hardware supports
> independent tags for each queue. "
> 
> As you now know, it doesn't.
> 
> TBH, I would be slightly surprised if any SCSI host supported independent
> tags per HW queue (ignoring these tri-mode types). The reason is that
> hisi_sas uses this tag as SAS SSP frame tag, and later used for TMF tag. If
> we sent different commands to a SCSI end device from different queues, then
> possibly non-unique tags and this would break the model. But maybe other
> SCSI host work differently.

When I tested some SCSI FC at MQ mode years ago, I did suspect if their tags
is hw queue wide, but result is really true.

Thanks,
Ming



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