Re: [PATCH 03/15] scsi: implement reserved command handling

["Asutosh Das (asd)" <asutoshd@codeaurora.org>]

On 11/25/2021 7:10 AM, Hannes Reinecke wrote:
> 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>
> ---
Reviewed-by: Asutosh Das <asutoshd@codeaurora.org>

>   drivers/scsi/hosts.c     |  3 +++
>   drivers/scsi/scsi_lib.c  |  9 ++++++++-
>   include/scsi/scsi_host.h | 22 +++++++++++++++++++++-
>   3 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index a539fa2fb221..8ee7a7279b6b 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -482,6 +482,9 @@ 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 6fbd36c9c416..e8f1025d0ed8 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1939,7 +1939,9 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
>   		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 = NUMA_NO_NODE;
>   	tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
> @@ -1964,6 +1966,9 @@ void scsi_mq_destroy_tags(struct Scsi_Host *shost)
>    * @nowait: do not wait for command allocation to succeed.
>    *
>    * Allocates a SCSI command for internal LLDD use.
> + * If 'nr_reserved_commands' is spectified by the host the
> + * command will be allocated from the reserved tag pool;
> + * otherwise the normal tag pool will be used.
>    */
>   struct scsi_cmnd *scsi_get_internal_cmd(struct scsi_device *sdev,
>   	int data_direction, bool nowait)
> @@ -1973,6 +1978,8 @@ struct scsi_cmnd *scsi_get_internal_cmd(struct scsi_device *sdev,
>   	blk_mq_req_flags_t flags = 0;
>   	int op;
>   
> +	if (sdev->host->nr_reserved_cmds)
> +		flags |= BLK_MQ_REQ_RESERVED;
>   	if (nowait)
>   		flags |= BLK_MQ_REQ_NOWAIT;
>   	op = (data_direction == DMA_TO_DEVICE) ?
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 6f49a8940dc4..7512d97aceb4 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -367,10 +367,19 @@ 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
> +	 * 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
> @@ -608,6 +617,11 @@ 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;
> @@ -626,6 +640,12 @@ 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;
>   
>   	/*
> 


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