Re: [PATCH v2 06/20] scsi: core: Add support for reserved tags

[John Garry <john.garry@huawei.com>]

On 19/11/2021 19:57, Bart Van Assche wrote:
> Allow SCSI LLDs to allocate reserved tags by passing the BLK_MQ_REQ_RESERVED
> flag to blk_mq_alloc_request().
> 
> Cc: Hannes Reinecke <hare@suse.de>
> Cc: John Garry <john.garry@huawei.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org > ---
>   drivers/scsi/scsi_lib.c  | 4 +++-
>   include/scsi/scsi_host.h | 9 ++++++++-
>   2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 59c3c4fbcfc0..44489ddc646c 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1925,6 +1925,7 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
>   {
>   	unsigned int cmd_size, sgl_size;
>   	struct blk_mq_tag_set *tag_set = &shost->tag_set;
> +	unsigned int reserved_tags = shost->hostt->reserved_tags;
>   
>   	sgl_size = max_t(unsigned int, sizeof(struct scatterlist),
>   				scsi_mq_inline_sgl_size(shost));
> @@ -1940,7 +1941,8 @@ 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 + reserved_tags;
> +	tag_set->reserved_tags = reserved_tags;
>   	tag_set->cmd_size = cmd_size;
>   	tag_set->numa_node = NUMA_NO_NODE;
>   	tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 72e1a347baa6..ec0f7705e06a 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -367,10 +367,17 @@ struct scsi_host_template {

why no field in struct Scsi_Host?

>   	/*
>   	 * 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. Does
> +	 * not include @reserved_tags.
>   	 */
>   	int can_queue;
>   
> +	/*
> +	 * Number of tags to reserve.


> A reserved tag can be allocated by passing
> +	 * the BLK_MQ_REQ_RESERVED flag to blk_get_request().

I don't see why we need this comment.

> +	 */
> +	unsigned reserved_tags;

I thought that unsigned int was preferred

> +
>   	/*
>   	 * In many instances, especially where disconnect / reconnect are
>   	 * supported, our host also has an ID on the SCSI bus.  If this is
> .
>