Re: [PATCH V4 1/2] scsi: core: avoid host-wide host_busy counter for scsi_mq

[Ming Lei <ming.lei@redhat.com>]

On Fri, Oct 25, 2019 at 09:58:16AM +0100, John Garry wrote:
> 
> > > In scsi_host.h, we have for scsi_host_template.can_queue: "It is set to the
> > > maximum number of simultaneous commands a given host adapter will accept.",
> > > so that should be honoured.
> > 
> 
> Hi Ming,
> 
> > That words should have been changed to:
> > 
> > "It is set to the maximum number of simultaneous commands a given host adapter's
> > hw queue will accept."
> 
> I find this definition misleading. As you know, some MQ SAS HBAs can accept
> .can_queue commands on a given hw queue, but can still only accept
> .can_queue commands over all hw queues.

I don't know there are such MQ HBA driver in tree, at least that is the
current blk-mq/scsi-mq model: each hw queue has its own independent
tags, so there can't be the limit for MQ HBA, which should allow to
accept (.can_queue * nr_hw_queues) commands. And I did hear people
complains bad performance caused by the atomic .host_busy counter.

If you are talking about the current SQ(from blk-mq or scsi-mq viewpoint) HBA
which has multiple reply queue(HPSA, hisilicon SAS, mpt3sas, and megaraid_sas),
they are just the special type. According to scsi-mq's model, they should
belong to SQ HBA.

> 
> > 
> > > 
> > > And Scsi_host.nr_hw_queues: "it is assumed that each hardware queue has a
> > > queue depth of can_queue. In other words, the total queue depth per host is
> > > nr_hw_queues * can_queue."
> > 
> > The above is correct.
> > 
> > > 
> > > I don't read "total queue depth per host" same as "maximum number of
> > > simultaneous commands a given host adapter will accept". If anything, the
> > > nr_hw_queues comment is ambiguous.
> > > 
> > > > 
> > > > The point is simple, because each hw queue has its own independent tags,
> > > > that is why I mentioned your Hisilicon SAS can't be converted to MQ
> > > > easily cause this hardware has only single shared tags.
> > > 
> > > Please be aware that HiSilicon SAS HW would not be unique for SCSI HBAs in
> > > this regard, in that the unique hostwide tag is not just for HBA HW IO
> > > management, but also is used as the tag for SCSI TMFs.
> > 
> > Right.
> > 
> > > 
> > > Just checking mpt3sas seems similar:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/mpt3sas/mpt3sas_scsih.c#n2918
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/mpt3sas/mpt3sas_base.c#n3546
> > 
> > Not only mpt3sas, there are also HPSA and more. And these drivers have to
> > support single hw queue of blk-mq, instead of real MQ. And the reason is that
> > these HBA has single tags.
> 
> We should be able to do better than that.
> 
> For a start, at least doesn't the check you remove in scsi_host_is_busy()
> limit commands the HBA accepts to .can_queue?

As I mentioned above, that is current blk-mq/scsi-mq's model, each hw
queue has its own independent tags, so the check really doesn't make
sense.

> 
> And if you make the change in this patch, then the changes to improve blk-mq
> for CPU hotplug are pointless, as we can't change the SAS HBAs to expose
> multiple queues.

No, just the small number of special type SCSI HBAs with multiple reply queue
and single tags can't benefit from the patchset of 'improve blk-mq for CPU hotplug',
and all other normal MQ device/drivers do get improved wrt. CPU hotplug.

We have tried hosttags approach for the several drivers, but looks it is
too messy. Given there are only 3 or 4 such device, we still can improve
them via driver private approach in future if no generic way is doable. 


Thanks,
Ming