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
