Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

[John Garry]

On 11/10/2018 14:32, Ming Lei wrote:
> On Thu, Oct 11, 2018 at 02:12:11PM +0100, John Garry wrote:
> > On 11/10/2018 11:15, Christoph Hellwig wrote:
> > > On Thu, Oct 11, 2018 at 10:59:11AM +0100, John Garry wrote:
> > > > > blk-mq tags are always per-host (which has actually caused problems for
> > > > > ATA, which is now using its own per-device tags).
> > > >
> > > > So, for example, if Scsi_host.can_queue = 2048 and Scsi_host.nr_hw_queues =
> > > > 16, then rq tags are still in range [0, 2048) for that HBA, i.e. invariant
> > > > on queue count?
> > >
> > > Yes, if can_queue is 2048 you will gets tags from 0..2047.
> >
> > I should be clear about some things before discussing this further. Our
> > device has 16 hw queues. And each command we send to any queue in the device
> > must have a unique tag across all hw queues for that device, and should be
> > in the range [0, 2048) - it's called an IPTT. So Scsi_host.can_queue = 2048.
>
> Could you describe a bit about IPTT?

IPTT is an "Initiator Tag". It is a tag to map to the context of a hw 
queue command. It is related to SAS protocol Initiator Port tag. I think 
that most SAS HBAs have a similar concept.

IPTTs are limited, and must be recycled when an IO completes. Our hw 
supports upto 2048. So we have a limit of 2048 commands issued at any 
point in time.

Previously we had been managing IPTT in LLDD, but found rq tag can be 
used as IPTT (as in 6/7), to gave a good performance boost.

> Looks like the 16 hw queues are like reply queues in other drivers,
> such as megara_sas, but given all the 16 reply queues share one tagset,
> so the hw queue number has to be 1 from blk-mq's view.
>
> > However today we only expose a single queue to upper layer (for unrelated
> > LLDD error handling restriction). We hope to expose all 16 queues in future,
> > which is what I meant by "enabling SCSI MQ in the driver". However, with
> > 6/7, this creates a problem, below.
>
> If the tag of each request from all hw queues has to be unique, you
> can't expose all 16 queues.

Well we can if we generate and manage the IPTT in the LLDD, as we had 
been doing. If we want to use the rq tag - which 6/7 is for - then we can't.

> > > IFF you device needs different tags for different queues it can use
> > > the blk_mq_unique_tag heper to generate unique global tag.
> >
> > So this helper can't help, as fundamentially the issue is "the tag field in
> > struct request is unique per hardware queue but not all all hw queues".
> > Indeed blk_mq_unique_tag() does give a unique global tag, but cannot be used
> > for the IPTT.
> >
> > OTOH, We could expose 16 queues to upper layer, and drop 6/7, but we found
> > it performs worse.
>
> We discussed this issue before, but not found a good solution yet for
> exposing multiple hw queues to blk-mq.

I just think that it's unfortunate that enabling blk-mq means that the 
LLDD loses this unique tag across all queues in range [0, 
Scsi_host.can_queue), so much so that we found performance better by not 
exposing multiple queues and continuing to use single rq tag...

> However, we still get good performance in case of none scheduler by the
> following patches:
>
> 8824f62246be blk-mq: fail the request in case issue failure
> 6ce3dd6eec11 blk-mq: issue directly if hw queue isn't busy in case of 'none'

I think that these patches would have been included in our testing. I 
need to check.

> Thanks,
> Ming


Thanks,
John

> .