[RFC 3/3] nvme: create special request queue for cdev

[Keith Busch]

On Wed, May 03, 2023 at 07:04:57AM +0200, Christoph Hellwig wrote:
> On Mon, May 01, 2023 at 08:33:06AM -0700, Keith Busch wrote:
> > From: Keith Busch <kbusch at kernel.org>
> > 
> > The cdev only services passthrough commands which don't merge, track
> > stats, or need accounting. Give it a special request queue with all
> > these options cleared so that we're not adding overhead for it.
> 
> Why can't we always skip these for passthrough commands on any queue
> with a little bit of core code?

A special queue without a gendisk was a little easier to ensure
it's not subscribed to any rq_qos features, iolatency, wbt,
blkthrottle. We might be able to add more blk_rq_is_passthrough()
for things we want to skip and get the same benefit. I'll look into
it.