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 11:13:12AM +0100, John Garry wrote:
> On 25/10/2019 10:43, Ming Lei wrote:
> > 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."
> > > 
> 
> Hi Ming,
> 
> > > 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,
> 
> HiSilicon SAS HBA can accept 4096 commands on any given hw queue but can
> also only accept 4096 commands over all queues simultaneously.  In fact, the
> hw queue depth is configurable.
> 
> That's why I think that the definition is misleading.

That is why we call HiSilicon SAS is SQ device, still from
blk-mq/scsi-mq viewpoint, :-)

> 
> I think I sound like a broken record now :)
> 
> 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.
> > 
> 
> ok, seems reasonable for now.
> 
> > 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.
> > 
> 
> TBH, I'm not sure on the group of SCSI drivers which could benefit then.
> 
> As I see, only qla2xxx driver sets Scsi_host.nr_hw_queues and also uses
> pci_alloc_irq_vectors_affinity().

Other non-SCSI drivers can benefit from that patchset too, such as NVMe.

Except for qla2xxx, there are also lpfc, virtio-scsi and smartpqi, and
there will be more since I heard that new version of one other popular
SCSI HBA will support real MQ.

> 
> > 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.
> 
> ok, I'd rather not go on this path - you may say I'm digging my head in the
> sand. Anyway, I'll continue to support the 'improve blk-mq for CPU hotplug'
> effort.

I meant that way is the last straw, :-)


Thanks,
Ming