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

[Hannes Reinecke]

On 6/15/22 09:35, John Garry wrote:
> On 15/06/2022 00:43, Damien Le Moal wrote:
> > On 6/15/22 03:20, Bart Van Assche wrote:
> > > On 6/13/22 00:01, Damien Le Moal wrote:
> > > > On 6/9/22 19:29, John Garry wrote:
> > > > > +    /*
> > > > > +     * 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;
> > >
> > > +1 for Damien's request. I also prefer to keep can_queue as the maximum
> > > queue depth, whether or not nr_reserved_cmds has been set.
> >
> > For non SATA drives, I still think that is a good idea. However, for 
> > SATA,
> > we always have the internal tag command that is special. With John's
> > change, it would have to be reserved but that means we are down to 31 max
> > QD,
>
> My intention is to keep regular tag depth at 32 for SATA. We add an 
> extra tag as a reserved tag. Indeed, this is called a 'tag', but it's 
> just really the placeholder for what will be the ATA_TAG_INTERNAL request.
>
> About how we set scsi_host.can_queue, in this series we set .can_queue 
> as max regular tags, and the handling is as follows:
>
> scsi_mq_setup_tags():
> tag_set->queue_depth = shost->can_queue + shost->nr_reserved_cmds
> tag_set->reserved_tags = shost->nr_reserved_cmds
>
> So we honour the rule that blk_mq_tag_set.queue_depth is the total tag 
> depth, including reserved.
>
> Incidentally I think Christoph prefers to keep .can_queue at total max 
> tags including reserved:
> https://lore.kernel.org/linux-scsi/337339b7-6f4a-a25c-f11c-7f701b42d6a8@xxxxxxx/ 
>
>
> > so going backward several years... That internal tag for ATA does not
> > need to be reserved since this command is always used when the drive is
> > idle and no other NCQ commands are on-going.
>
> So do you mean that ATA_TAG_INTERNAL qc is used for other commands apart 
> from internal commands?
Well.

The problem is that 'ATA_TAG_INTERNAL' currently is overloaded to
a) signal internal commands
b) 'magic' tag when looking up commands

My proposal would be to separate these use-cases, and use a flag (eg 
ATA_QCFLAG_INTERNAL) to determine internal commands.

The we'll be needing an internal tag-lookup map
(NCQ tag -> blk-mq tag) for ata_qc_from_tag() to retrieve the command 
corresponding to a driver tag.

I guess we'll need that anyway with libsas, as there we're working with 
a budget tag which has no relationship with the NCQ tag whatsoever ...

But with that we can kill 'ATA_TAG_INTERNAL', and let the driver figure 
out how to allocate internal tags.

Cheers,

Hannes
--
Dr. Hannes Reinecke		           Kernel Storage Architect
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