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

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