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

[Hannes Reinecke]

On 6/13/22 09:01, Damien Le Moal wrote:
> On 6/9/22 19:29, John Garry wrote:
> > From: Hannes Reinecke <hare@xxxxxxx>
> >
> > 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@xxxxxxx>
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> >   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.
I had been thinking about this for quite a while, and figured that the 
'reserved' commands model from blk-mq doesn't fit nicely with the SATA 
protocol.

So my idea for SATA is simply _not_ to use reserved tags.
Any TMF functions (or the equivalent thereof) should always be sent as 
non-NCQ commands. And when doing so we're back to QD=1 on SATA anyway, 
so there _must_ be tags available. Consequently the main reason for 
having reserved tags (namely to guarantee that tags are available for 
TMF) doesn't apply here.

Which is why in my initial patchset I've always been referrring to 
'internal' commands, and drivers could select if the 'internal' commands 
are mappend on reserved tags or not.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
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