Date:   Thu, 11 Oct 2018 15:07:33 +0100
From:   John Garry <john.garry@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     Christoph Hellwig <hch@...radead.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        <jejb@...ux.vnet.ibm.com>, <linuxarm@...wei.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        chenxiang <chenxiang66@...ilicon.com>
Subject: Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

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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