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

[John Garry]

On 12/11/2019 13:54, John Garry wrote:
> > > 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.

Scratch that. I have seen the issue here also.

Thanks,
John

> 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
> .