__i915_spin_request() sucks

[Chris Wilson]

On Fri, Nov 13, 2015 at 09:22:52AM -0700, Jens Axboe wrote:
> On 11/13/2015 09:13 AM, Mike Galbraith wrote:
> >On Fri, 2015-11-13 at 08:36 -0700, Jens Axboe wrote:
> >>Previous patch was obvious pre-coffee crap, this should work for using
> >>ktime to spin max 1usec.
> >
> >1us seems a tad low.  I doubt the little wooden gears and pulleys of my
> >core2 Toshiba Satellite lappy can get one loop ground out in a usec :)
> 
> Maybe it is, it's based off the original intent of the function,
> though. See the original commit referenced.

I've been looking at numbers from one laptop and I can set the timeout
at 2us before we see a steep decline in what is more or less synchronous
request handling (which affects a variety of rendering workloads).

Looking around, other busy loops seem to use local_clock() (i.e. rdstcll
with a fair wind). Is that worth using here?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre