Messages in this thread | | | Subject | Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch | From | John Garry <> | Date | Thu, 11 Oct 2018 15:07:33 +0100 |
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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
> > . >
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