On Mon, 3 Apr 2017, 8:20am, Bart Van Assche wrote:

> On Mon, 2017-04-03 at 09:29 +0200, Hannes Reinecke wrote:
> > On 04/03/2017 08:37 AM, Arun Easi wrote:
> > > If the above is true, then for a LLD to get tag# within it's max-tasks 
> > > range, it has to report max-tasks / number-of-hw-queues in can_queue, and 
> > > in the I/O path, use the tag and hwq# to arrive at a index# to use. This, 
> > > though, leads to a poor use of tag resources -- queue reaching it's 
> > > capacity while LLD can still take it.
> >
> > Shared tag sets continue to dog the block-mq on 'legacy' (ie non-NVMe)
> > HBAs. ATM the only 'real' solution to this problem is indeed have a
> > static split of the entire tag space by the number of hardware queues.
> > With the mentioned tag-starvation problem.
> 
> Hello Arun and Hannes,
> 
> Apparently the current blk_mq_alloc_tag_set() implementation is well suited
> for drivers like NVMe and ib_srp but not for traditional SCSI HBA drivers.
> How about adding a BLK_MQ_F_* flag that tells __blk_mq_alloc_rq_maps() to
> allocate a single set of tags for all hardware queues and also to add a flag
> to struct scsi_host_template such that SCSI LLDs can enable this behavior?
> 

Hi Bart,

This would certainly be beneficial in my case. Moreover, it certainly 
makes sense to move the logic up where multiple drivers can leverage. 

Perhaps, use percpu_ida* interfaces to do that, but I think I read 
somewhere that, it is not efficient (enough?) and is the reason to go the 
current way for block tags.

Regards,
-Arun