Re: [PATCH] scsi: scsi_host_queue_ready: increase busy count early

[Martin Wilck <mwilck@suse.com>]

On Tue, 2021-02-02 at 20:04 +0000, Don.Brace@microchip.com wrote:
> -----Original Message-----
> From: John Garry [mailto:john.garry@huawei.com] 
> Subject: Re: [PATCH] scsi: scsi_host_queue_ready: increase busy count
> early
> 
> 
> Confirmed my suspicions - it looks like the host is sent more
> commands than it can handle. We would need many disks to see this
> issue though, which you have.
> 
> So for stable kernels, 6eb045e092ef is not in 5.4 . Next is 5.10, and
> I suppose it could be simply fixed by setting .host_tagset in scsi
> host template there.
> 
> Thanks,
> John
> --
> Don: Even though this works for current kernels, what would chances
> of this getting back-ported to 5.9 or even further?
> 
> Otherwise the original patch smartpqi_fix_host_qdepth_limit would
> correct this issue for older kernels.

True. However this is 5.12 material, so we shouldn't be bothered by
that here. For 5.5 up to 5.9, you need a workaround. But I'm unsure
whether smartpqi_fix_host_qdepth_limit would be the solution.
You could simply divide can_queue by nr_hw_queues, as suggested before,
or even simpler, set nr_hw_queues = 1.

How much performance would that cost you?

Distribution kernels would be yet another issue, distros can backport
host_tagset and get rid of the issue.

Regards
Martin