[Intel-gfx] [PATCH 01/11] drm/i915/execlists: Avoid kicking the submission too early for rescheduling

[Chris Wilson]

Quoting Mika Kuoppala (2018-03-27 12:34:31)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > If the request is still waiting on external fences, it has not yet been
> > submitted to the HW queue and so we can forgo kicking the submission
> > tasklet when re-evaluating its priority.
> >
> > This should have no impact other than reducing the number of tasklet
> > wakeups under signal heavy workloads (e.g. switching between engines).
> >
> > v2: Use prebaked container_of()
> >
> > References: f6322eddaff7 ("drm/i915/preemption: Allow preemption between submission ports")
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Michał Winiarski <michal.winiarski at intel.com>
> > Cc: Michel Thierry <michel.thierry at intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_lrc.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index b4ab06b05e58..104b69e0494f 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -1051,12 +1051,16 @@ static void queue_request(struct intel_engine_cs *engine,
> >       list_add_tail(&pt->link, &lookup_priolist(engine, pt, prio)->requests);
> >  }
> >  
> > -static void submit_queue(struct intel_engine_cs *engine, int prio)
> > +static void __submit_queue(struct intel_engine_cs *engine, int prio)
> >  {
> > -     if (prio > engine->execlists.queue_priority) {
> >               engine->execlists.queue_priority = prio;
> >               tasklet_hi_schedule(&engine->execlists.tasklet);
> > -     }
> > +}
> > +
> > +static void submit_queue(struct intel_engine_cs *engine, int prio)
> > +{
> > +     if (prio > engine->execlists.queue_priority)
> > +             __submit_queue(engine, prio);
> 
> You did this...
> 
> >  }
> >  
> >  static void execlists_submit_request(struct i915_request *request)
> > @@ -1189,7 +1193,10 @@ static void execlists_schedule(struct i915_request *request, int prio)
> >                       __list_del_entry(&pt->link);
> >                       queue_request(engine, pt, prio);
> >               }
> > -             submit_queue(engine, prio);
> > +
> > +             if (prio > engine->execlists.queue_priority &&
> > +                 i915_sw_fence_done(&pt_to_request(pt)->submit))
> > +                     __submit_queue(engine, prio);
> 
> ..to have explicit priority comparison on submit callsite I gather.
> Or is there some other reason?

No, just because I wanted both checks in this case. On the other path
i915_sw_fence_done() isn't technically true as we are in process of
performing the i915_sw_fence callback, so just i915_sw_fence_signaled().
But we don't want to use i915_sw_fence_signaled() here as I don't want
to think about the race. :)

So since prio > engine.queue_priority should be cheaper than loading the
cacheline for the request->submit.flags, I wanted that tested first as
it will only fire, at most, once per engine.
-Chris