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

[Hannes Reinecke <hare@suse.de>]

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?
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
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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