Re: [PATCH RFC v3 03/22] scsi: core: Implement reserved command handling

[Hannes Reinecke]

On 10/27/22 03:18, Damien Le Moal wrote:
> On 10/25/22 19:17, 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>
> > #jpg: Set tag_set->queue_depth = shost->can_queue, and not
> > = shost->can_queue + shost->nr_reserved_cmds;
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> >   drivers/scsi/hosts.c     |  3 +++
> >   drivers/scsi/scsi_lib.c  |  2 ++
> >   include/scsi/scsi_host.h | 15 ++++++++++++++-
> >   3 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> > index 12346e2297fd..db89afc37bc9 100644
> > --- a/drivers/scsi/hosts.c
> > +++ b/drivers/scsi/hosts.c
> > @@ -489,6 +489,9 @@ 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;
> > +
>
> Nit: the if is not really necessary I think. But it does not hurt.
Yes, we do.
Not all HBAs are able to figure out the number of reserved commands 
upfront; some modify that based on the PCI device used etc.
So I'd keep it for now.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew
Myers, Andrew McDonald, Martje Boudien Moerman