Re: [PATCH 4/4] scsi: core: don't limit per-LUN queue depth for SSD

[Ming Lei <ming.lei@redhat.com>]

On Wed, Nov 20, 2019 at 12:00:19PM -0500, Ewan D. Milne wrote:
> On Wed, 2019-11-20 at 11:05 +0100, Hannes Reinecke wrote:
> > 
> > Hmm.
> > 
> > I must admit I patently don't like this explicit dependency on
> > blk_nonrot(). Having a conditional counter is just an open invitation to
> > getting things wrong...
> > 
> 
> This concerns me as well, it seems like the SCSI ML should have it's
> own per-device attribute if we actually need to control this per-device
> instead of on a per-host or per-driver basis.  And it seems like this
> is something that is specific to high-performance drivers, so changing
> the way this works for all drivers seems a bit much.
> 
> Ordinarily I'd prefer a host template attribute as Sumanesh proposed,
> but I dislike wrapping the examination of that and the queue flag in
> a macro that makes it not obvious how the behavior is affected.
> (Plus Hannes just submitted submitted the patches to remove .use_cmd_list,
> which was another piece of ML functionality used by only a few drivers.)
> 
> Ming's patch does freeze the queue if NONROT is changed by sysfs, but
> the flag can be changed by other kernel code, e.g. sd_revalidate_disk()
> clears it and then calls sd_read_block_characteristics() which may set
> it again.  So it's not clear to me how reliable this is.

The queue freeze is applied in sd_revalidate_disk() too, isn't it?

Thanks,
Ming