[Intel-gfx] [PATCH] drm/i915/execlists: Set queue priority from secondary port

[Chris Wilson]

Quoting Chris Wilson (2018-03-29 10:39:00)
> We can refine our current execlists->queue_priority if we inspect
> ELSP[1] rather than the head of the unsubmitted queue. Currently, we use
> the unsubmitted queue and say that if a subsequent request is more than
> important than the current queue, we will rerun the submission tasklet
> to evaluate the need for preemption. However, we only want to preempt if
> we need to jump ahead of a currently executing request in ELSP. The
> second reason for running the submission tasklet is amalgamate requests
> into the active context on ELSP[0] to avoid a stall when ELSP[0] drains.
> (Though repeatedly amalgamating requests into the active context and
> triggering many lite-restore is off question gain, the goal really is to
> put a context into ELSP[1] to cover the interrupt.) So if instead of
> looking at the head of the queue, we look at the context in ELSP[1] we
> can answer both of the questions more accurately -- we don't need to
> rerun the submission tasklet unless our new request is important enough
> to feed into, at least, ELSP[1].
> 
> 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>

Has anyone had a chance to think about this yet?
-Chris

> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 654634254b64..e6d3ee4ca0b2 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -698,7 +698,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>                         kmem_cache_free(engine->i915->priorities, p);
>         }
>  done:
> -       execlists->queue_priority = rb ? to_priolist(rb)->priority : INT_MIN;
> +       execlists->queue_priority =
> +               port != execlists->port ? rq_prio(last) : INT_MIN;
>         execlists->first = rb;
>         if (submit)
>                 port_assign(port, last);
> -- 
> 2.16.3
>