[Intel-gfx] [PATCH i-g-t] i915/gem_ctx_engine: Prevent preemption

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-06-19 07:32:05)
> 
> On 18/06/2019 21:21, Chris Wilson wrote:
> > In order to pin the engine as busy, we have to prevent the kernel from
> > executing other independent work ahead of our plug, so tell the spinner
> > to not allow preemption.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >   tests/i915/gem_ctx_engines.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tests/i915/gem_ctx_engines.c b/tests/i915/gem_ctx_engines.c
> > index 3ecade417..d47cbdd7c 100644
> > --- a/tests/i915/gem_ctx_engines.c
> > +++ b/tests/i915/gem_ctx_engines.c
> > @@ -283,7 +283,8 @@ static void execute_one(int i915)
> >   
> >                       spin = igt_spin_new(i915,
> >                                           .ctx = param.ctx_id,
> > -                                         .engine = 0);
> > +                                         .engine = 0,
> > +                                         .flags = IGT_SPIN_NO_PREEMPTION);
> >   
> >                       igt_debug("Testing with map of %d engines\n", i + 1);
> >                       memset(&engines.engines, -1, sizeof(engines.engines));
> > 
> 
> The no-op batch preempts the spinner? How does that affect the busy 
> check on the no-op batch?

We are checking the second batch in the pipeline, expecting it to be
blocked by the already executing spinner on the engine of interest.
There is no dependency between the two batches, just order of
submission, hence the kernel ends up executing the second no-op batch
after the first batch's timeslice expired.
-Chris