drm/i915/execlists: Don't merely skip submission if maybe timeslicing

Normally, we try and skip submission if ELSP[1] is filled. However, we
may desire to enable timeslicing due to the queue priority, even if
ELSP[1] itself does not require timeslicing. That is the queue is equal
priority to ELSP[0] and higher priority then ELSP[1]. Previously, we
would wait until the context switch to preempt the current ELSP[1], but
with timeslicing, we want to preempt ELSP[0] and replace it with the
queue.

In writing the test case, it become quickly apparent that we were also
suppressing the tasklet during promotion and so failing to notice when
the queue started requiring timeslicing.

Fixes: 2229adc81380 ("drm/i915/execlist: Trim immediate timeslice expiry")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018072027.31948-1-chris@chris-wilson.co.uk

1 files changed, 15 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 7b84ebca2901..0ca40f6bf08c 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -177,9 +177,22 @@ static inline int rq_prio(const struct i915_request *rq)
 	return rq->sched.attr.priority | __NO_PREEMPTION;
 }
 
+static inline bool need_preempt(int prio, int active)
+{
+	/*
+	 * Allow preemption of low -> normal -> high, but we do
+	 * not allow low priority tasks to preempt other low priority
+	 * tasks under the impression that latency for low priority
+	 * tasks does not matter (as much as background throughput),
+	 * so kiss.
+	 */
+	return prio >= max(I915_PRIORITY_NORMAL, active);
+}
+
 static void kick_submission(struct intel_engine_cs *engine, int prio)
 {
-	const struct i915_request *inflight = *engine->execlists.active;
+	const struct i915_request *inflight =
+		execlists_active(&engine->execlists);
 
 	/*
 	 * If we are already the currently executing context, don't
@@ -188,7 +201,7 @@ static void kick_submission(struct intel_engine_cs *engine, int prio)
 	 * tasklet, i.e. we have not change the priority queue
 	 * sufficiently to oust the running context.
 	 */
-	if (!inflight || !i915_scheduler_need_preempt(prio, rq_prio(inflight)))
+	if (!inflight || !need_preempt(prio, rq_prio(inflight)))
 		return;
 
 	tasklet_hi_schedule(&engine->execlists.tasklet);