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

[John Garry]

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.

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.

> 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.

> But unless you actuall have multiple hardware queues that latter part
> is rather irrelevant to start with.
>
> .

Thanks,
John