Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 6/9/22 19:29, John Garry wrote:
> From: Hannes Reinecke <hare@suse.de>
> 
> Quite some drivers are using management commands internally, which
> typically use the same hardware tag pool (ie they are being allocated
> from the same hardware resources) as the 'normal' I/O commands.
> These commands are set aside before allocating the block-mq tag bitmap,
> so they'll never show up as busy in the tag map.
> The block-layer, OTOH, already has 'reserved_tags' to handle precisely
> this situation.
> So this patch adds a new field 'nr_reserved_cmds' to the SCSI host
> template to instruct the block layer to set aside a tag space for these
> management commands by using reserved tags.
> 
> Signed-off-by: Hannes Reinecke <hare@suse.de>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
>  drivers/scsi/hosts.c     |  3 +++
>  drivers/scsi/scsi_lib.c  |  6 +++++-
>  include/scsi/scsi_host.h | 22 +++++++++++++++++++++-
>  3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 8352f90d997d..27296addaf63 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -474,6 +474,9 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
>  	if (sht->virt_boundary_mask)
>  		shost->virt_boundary_mask = sht->virt_boundary_mask;
>  
> +	if (sht->nr_reserved_cmds)
> +		shost->nr_reserved_cmds = sht->nr_reserved_cmds;
> +
>  	device_initialize(&shost->shost_gendev);
>  	dev_set_name(&shost->shost_gendev, "host%d", shost->host_no);
>  	shost->shost_gendev.bus = &scsi_bus_type;
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 6ffc9e4258a8..f6e53c6d913c 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1974,8 +1974,12 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
>  	else
>  		tag_set->ops = &scsi_mq_ops_no_commit;
>  	tag_set->nr_hw_queues = shost->nr_hw_queues ? : 1;
> +
>  	tag_set->nr_maps = shost->nr_maps ? : 1;
> -	tag_set->queue_depth = shost->can_queue;
> +	tag_set->queue_depth =
> +		shost->can_queue + shost->nr_reserved_cmds;
> +	tag_set->reserved_tags = shost->nr_reserved_cmds;
> +
>  	tag_set->cmd_size = cmd_size;
>  	tag_set->numa_node = dev_to_node(shost->dma_dev);
>  	tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 59aef1f178f5..149dcbd4125e 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -366,10 +366,19 @@ struct scsi_host_template {
>  	/*
>  	 * This determines if we will use a non-interrupt driven
>  	 * or an interrupt driven scheme.  It is set to the maximum number
> -	 * of simultaneous commands a single hw queue in HBA will accept.
> +	 * of simultaneous commands a single hw queue in HBA will accept
> +	 * excluding internal commands.
>  	 */
>  	int can_queue;
>  
> +	/*
> +	 * This determines how many commands the HBA will set aside
> +	 * for internal commands. This number will be added to
> +	 * @can_queue to calcumate the maximum number of simultaneous

s/calcumate/calculate

But this is weird. For SATA, can_queue is 32. Having reserved commands,
that number needs to stay the same. We cannot have more than 32 tags.
I think keeping can_queue as the max queue depth with at most
nr_reserved_cmds tags reserved is better.

> +	 * commands sent to the host.
> +	 */
> +	int nr_reserved_cmds;
> +
>  	/*
>  	 * In many instances, especially where disconnect / reconnect are
>  	 * supported, our host also has an ID on the SCSI bus.  If this is
> @@ -602,6 +611,11 @@ struct Scsi_Host {
>  	unsigned short max_cmd_len;
>  
>  	int this_id;
> +
> +	/*
> +	 * Number of commands this host can handle at the same time.
> +	 * This excludes reserved commands as specified by nr_reserved_cmds.
> +	 */
>  	int can_queue;
>  	short cmd_per_lun;
>  	short unsigned int sg_tablesize;
> @@ -620,6 +634,12 @@ struct Scsi_Host {
>  	 */
>  	unsigned nr_hw_queues;
>  	unsigned nr_maps;
> +
> +	/*
> +	 * Number of reserved commands to allocate, if any.
> +	 */
> +	unsigned nr_reserved_cmds;
> +
>  	unsigned active_mode:2;
>  
>  	/*


-- 
Damien Le Moal
Western Digital Research