Re: [PATCH 6/6] scsi: hisi_sas: Expose multiple hw queues for v3 as experimental

[John Garry]

> > I mentioned in the thread "blk-mq: improvement on handling IO during CPU
> > hotplug" that I was using this series to test that patchset.
> >
> > So just with this patchset (and without yours), I get what looks like some
> > IO errors in the LLDD. The error is an underflow error. I can't figure out
> > what is the cause.

Hi Ming,

> Can you post the error log? Or interpret the 'underflow error' from hisi
> sas or scsi viewpoint?

The check here fails:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/libsas/sas_scsi_host.c?h=v5.4-rc7#n57

Indeed, no data is received.

> > I'm wondering if the SCSI command is getting corrupted someway.
>
> Why do you think the command is corrupted?

I considered that the underflow may occur if we were to clobber a SCSI 
command/request from another hctx and zero some fields, which is 
detected as an underflow. But that's just guessing.

However do I find if I set shost->can_queue = HISI_SAS_MAX_COMMANDS / 
#queues, then no issue. But maybe that's a coincidence. For this, total 
queue depth = HISI_SAS_MAX_COMMANDS. I don't see the impact of that.

I need to test that more.

> > > > +	if (expose_mq_experimental) {
> > > > +		shost->can_queue = HISI_SAS_MAX_COMMANDS;
> > > > +		shost->cmd_per_lun = HISI_SAS_MAX_COMMANDS;
> > > The above is contradictory with current 'nr_hw_queues''s meaning,
> > > see commit on Scsi_Host.nr_hw_queues.
> >
> > Right, so I am generating the hostwide tag in the LLDD. And the Scsi
> > host-wide host_busy counter should ensure that we don't pump too much IO to
> > the HBA.
>
> Even without the host-wide host_busy, your approach should work if you
> build the hisi sas tag correctly(uniquely), just not efficiently.

Yes, I do that.

 I'd
> suggest you to collect trace and observe if request with expected hisi sas
> tag is sent to hardware.

I can add some debug for that. What trace do you mean?

> BTW, the patch of 'scsi: core: avoid host-wide host_busy counter for scsi_mq'
> will be merged to v5.5 if everything is fine.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/mkp/scsi.git/commit/?h=5.5/scsi-queue&id=6eb045e092efefafc6687409a6fa6d1dabf0fb69

Yeah, it seems a good change.

Thanks,
John