Re: [PATCH 3/4] scsi: Support scsi_devices without a device wide limit

[Mike Christie <michael.christie@oracle.com>]

On 4/22/26 8:15 AM, Hannes Reinecke wrote:
> On 4/18/26 00:57, Mike Christie wrote:
>> For virtio-scsi, we export a wide variety of non-scsi devices like
>> NVMe (local and RDMA/TCP based) drives and block based devices using
>> ublk. And then it's common to have multiple high perf devices im a LVM
>> volume. The problem for these setups, is we can easily hit the 4096
>> scsi_device queue depth limit so we end up throttling IO in the guest
>> when the real device can handle more IO.
>>
>> In these situations we don't have a device wide limit that maps to
>> cmd_per_lun. We have per hw queue limits or on the host we are doing
>> more dynamic throttling. To allow for these types of devices, this
>> patch allows drivers to set SCSI_UNLIMITED_CMD_PER_LUN for the
>> cmd_per_lun. When set, we will then only be limited by the per hw
>> queue limits.
>>
>> Signed-off-by: Mike Christie <michael.christie@oracle.com>
>> ---
>>   drivers/scsi/hosts.c     |  5 +++--
>>   drivers/scsi/scsi_scan.c | 25 ++++++++++++++-----------
>>   include/scsi/scsi_host.h |  4 ++++
>>   3 files changed, 21 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
>> index e047747d4ecf..c93c59e847c5 100644
>> --- a/drivers/scsi/hosts.c
>> +++ b/drivers/scsi/hosts.c
>> @@ -238,8 +238,9 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
>>       }
>>         /* Use min_t(int, ...) in case shost->can_queue exceeds SHRT_MAX */
>> -    shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
>> -                   shost->can_queue);
>> +    if (shost->cmd_per_lun != SCSI_UNLIMITED_CMD_PER_LUN)
>> +        shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
>> +                       shost->can_queue);
>>         error = scsi_init_sense_cache(shost);
>>       if (error)
>> diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
>> index 7b11bc7de0e3..ecc3638c1909 100644
>> --- a/drivers/scsi/scsi_scan.c
>> +++ b/drivers/scsi/scsi_scan.c
>> @@ -352,18 +352,20 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
>>       if (scsi_device_is_pseudo_dev(sdev))
>>           return sdev;
>>   -    depth = sdev->host->cmd_per_lun ?: 1;
>> +    if (sdev->host->cmd_per_lun != SCSI_UNLIMITED_CMD_PER_LUN) {
>> +        depth = sdev->host->cmd_per_lun ?: 1;
>>   
> Why don't we use a simple flag in the host (or host template) to
> indicate that cmd_per_lun should be ignored?

That's fine with me. I don't need it per scsi_device, but had thought
someone might. We can always change it later to a scsi_device flag if
it comes up.


> I'm not in favour of using magic values for a setting which
> otherwise is a limit.
> Look to dev_loss_tmo as a bad example ...
> 
> Cheers,
> 
> Hannes