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
