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.
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?
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.
Thanks,
John
Thanks,
Ming
.
