[Intel-gfx] [PATCH igt] igt/gem_exec_latency: Wire up an interloper for preemption

[Chris Wilson]

Quoting Michał Winiarski (2017-10-26 11:52:31)
> On Thu, Oct 26, 2017 at 08:31:27AM +0100, Chris Wilson wrote:
> > For measuring the cost of preemption, inject a low priority spinner
> > between the two measurements; the difference between the preemption and
> > the normal dispatch includes both the cost of the spinner dispatch and
> > of preempting it. Close enough for us to estimate the cost of
> > preemption, though we don't measure the cost of preemption on the local
> > ring!
> 
> And as expected, we're seeing more delay with GuC, probably from worker
> scheduling delay (~2ms on my SKL if I'm reading the results correctly).

Don't look at bxt then ;) Another order or magnitude.

Most of that can be ascribed to using a worker, so we should be able to
pare it back somewhat. Just the idea that the GuC may take 10ms to
respond to a request is a bit disconcerting!

Do we have to wait for the ack from the preempt request? Could we farm
that off to some other poor task? Then we would be in a position to
avoid that mutex and process-context worker. Oh well, you probably
already have ideas and plans for replacing that mutex :)
-Chris