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

Ewan D. Milne
2019-11-21T19:19:39Z

On Thu, 2019-11-21 at 08:54 +0800, Ming Lei wrote:
> 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?
>

Yes, sorry, you are right, your patch does add this. But if anything else changes
the NONROT attribute for a queue associated with a SCSI device in the future
it would have to freeze the queue. Because the device_busy counter mechanism
would rely on it to work right. This isn't an obvious connection, it seems to me.

-Ewan