Re: [PATCH v4 6/6] scsi: core: Improve IOPS in case of host-wide tags

[Damien Le Moal <dlemoal@kernel.org>]

On 12/20/25 09:05, Bart Van Assche wrote:
> On 12/19/25 3:06 PM, Damien Le Moal wrote:
>> On 12/20/25 02:35, Bart Van Assche wrote:
>>> On 12/16/25 7:24 PM, Damien Le Moal wrote:
>>>> On 12/17/25 07:30, Bart Van Assche wrote:
>>>>> The SCSI core uses the budget map to restrict the number of commands
>>>>> that are in flight per logical unit. That limit check can be left out if
>>>>> host->cmd_per_lun >= host->can_queue and if the host tag set is shared
>>>>> across all hardware queues or if there is only one hardware queue  Since
>>>>
>>>> Missing a period at the end of the sentence (before Since). But more
>>>> importantly, this does not explain why the above is true, and frankly, I do not
>>>> see it...
>>> Hi Damien,
>>>
>>> The purpose of the SCSI device budget map is to prevent that the queue
>>> depth limit for that SCSI device is exceeded. If there is only a single
>>> hardware queue or there is a host-wide tag set and host->cmd_per_lun is
>>> identical to host->can_queue, it is not possible that the queue depth
>>> for a single SCSI device is exceeded and hence the SCSI device budget
>>> map is not needed.
>>
>> Still very confusing because as far as I understand things, a host is not
>> necessarily a LUN/block device (you can have several devices/LUNs on the same
>> host). So in general host->can_queue != device max queue depth. Also, I am not
>> entirely sure if host->cmd_per_lun and max queue depth of the LUN are the same
>> thing, given that SCSI does not define a maximum device queue depth...
> 
> Hi Damien,
> 
> There are important use cases, e.g. the UFS driver, where
> host->can_queue is identical to the maximum queue depth per logical
> unit. A single UFS device typically supports multiple logical units.

I get the use case aspect of this. But the above still does not clearly explains
things. E.g.: "host->can_queue is identical to the maximum queue depth per
logical unit" -> As I mentioned, SCSI does not define/advertize a maximum queue
depth per LU (beside the transport defined maximum of course). So Is this
something that UFS defines outside of SCSI/SBC ? Also, for UFS, is it always one
host per LU ? (that would be odd, the "device" here should be the host and you
say it can have multiple LUs).

But if I understand this correctly, you are saying that a UFS device is like
SATA and can_queue == device max queue depth, so we are always guaranteed that
if you can allocate a tag, you will be able to issue the command, right ?

>>> Please help with reviewing the ATA patch in this series.
>>
>> For AHCI, we are dealing with single queue devices, always. For this case, I do
>> not think that the scsi budget is needed since we will always have scsi tag ==
>> ATA tag, between 0 and 31. So if you can allocate a tag, you can always submit
>> the command.
>>
>> But for libsas HBAs, I am not sure at all if what you did is solid/works,
>> because I still do not understand it. Please provide more detailed explanations
>> in your code (comments) and commit messages to better explain what you are doing
>> is safe.
> 
> I plan to modify scsi_needs_budget_map() in patch 6/6 such that SCSI
> hosts that define a .change_queue_depth method and/or that set
> .track_queue_depth. This will prevent that this optimization applies to
> libsas HBAs. From include/scsi/libsas.h:
> 
> #define __LIBSAS_SHT_BASE						\
> 	[ ... ]
> 	.change_queue_depth		= sas_change_queue_depth,	\
> 	[ ... ]

.change_queue_depth is defined for AHCI too, with ata_scsi_change_queue_depth().

I am still not sure what you are trying to say here and what this proposed
change will do.

> 
> Thanks,
> 
> Bart.


-- 
Damien Le Moal
Western Digital Research