[PATCH 3/3] block: Polling completion performance optimization

[Christoph Hellwig]

On Thu, Dec 21, 2017 at 02:34:00PM -0700, Keith Busch wrote:
> It would be nice, but the driver doesn't know a request's completion
> is going to be a polled.

We can trivially set a REQ_POLL bit.  In fact my initial patch kit had
those on the insitence of Jens, but then I removed it because we had
no users for it.

> Even if it did, we don't have a spec defined
> way to tell the controller not to send an interrupt with this command's
> compeletion, which would be negated anyway if any interrupt driven IO
> is mixed in the same queue.

Time to add such a flag to the spec then..

> We could possibly create a special queue
> with interrupts disabled for this purpose if we can pass the HIPRI hint
> through the request.

Eww.  That means we double our resource usage both in the host and
the device, which isn't going to be pretty.