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 <[email protected]>
---
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
[email protected] +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
