Re: [RFC PATCH 0/2] block,scsi: support host-wide tagset

[Omar Sandoval <osandov@osandov.com>]

On Tue, Apr 04, 2017 at 02:07:43PM +0200, Hannes Reinecke wrote:
> Hi all,
> 
> as discussed recently most existing HBAs have a host-wide tagset which
> does not map easily onto the per-queue tagset model of block mq.
> This patchset implements a flag BLK_MQ_F_GLOBAL_TAGS for block-mq, which
> enables the use of a shared tagset for all hardware queues.
> The second patch adds a flag 'host_tagset' to the SCSI host template,
> which allows drivers to enable the use of the global tagset.
> 
> This patchset probably has some performance implications as
> there is a quite high probability of cache-bouncing when allocating
> tags. Also I'm not quite sure if the implemented tagset sharing
> is the correct way to handle things.
> So this can be considered an RFC.
> 
> As usual, comments and reviews are welcome.

Hi, Hannes,

blk-mq already supports a shared tagset, and scsi-mq already uses that.
When we initialize a request queue, we add it to a tagset with
blk_mq_add_queue_set(), where we automatically mark the tagset as shared
if there is more than one queue using it. What does this do that
BLK_MQ_F_TAG_SHARED doesn't cover?