[Intel-gfx] [PATCH 02/10] drm/i915/gt: Always check to enable timeslicing if not submitting

[Chris Wilson]

Quoting Mika Kuoppala (2020-06-05 16:20:34)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > We may choose not to submit for a number of reasons, yet not fill both
> > ELSP. In which case we must start timeslicing (there will be no ACK
> > event on which to hook the start) if the queue would benefit from the
> > currently active context being evicted.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_lrc.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index 92c3368ffcbd..d55a5e0466e5 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -2362,10 +2362,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >                               if (last->context == rq->context)
> >                                       goto done;
> >  
> > -                             if (i915_request_has_sentinel(last)) {
> > -                                     start_timeslice(engine, rq_prio(rq));
> > +                             if (i915_request_has_sentinel(last))
> >                                       goto done;
> > -                             }
> >  
> >                               /*
> >                                * If GVT overrides us we only ever submit
> > @@ -2446,6 +2444,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >               set_preempt_timeout(engine, *active);
> >               execlists_submit_ports(engine);
> >       } else {
> > +             start_timeslice(engine, execlists->queue_priority_hint);
> 
> If we ended up with same set of request, we want to skip submitting.
> But why would we want to skip timeslicing?

Because we have already submitted the exact same pair of requests
and so there will a be a set_timeslice() either pending or have taken
place. In particular, we wanted to stop timeslicing if after a timeslice
expiry we submitted exactly the same requests as before the timelice --
we know that until the arrival of a new request that there is no need
for a new timeslice, that will just result in the same pair being
submitted in order each time.
-Chris