drm/i915: Don't set queue_priority_hint if we don't kick the submission

If we change the priority of the active context, then it has no impact
on the decision of whether to preempt the active context -- we don't
preempt the context with itself. In this situation, we elide the tasklet
rescheduling and should *not* be marking up the queue_priority_hint as
that may mask a later submission where we decide we don't have to kick
the tasklet as a higher priority submission is pending (spoiler alert,
it was not).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021080226.537-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_scheduler.c

@@ -189,22 +189,34 @@ static inline bool need_preempt(int prio, int active)
 	return prio >= max(I915_PRIORITY_NORMAL, active);
 }
 
-static void kick_submission(struct intel_engine_cs *engine, int prio)
+static void kick_submission(struct intel_engine_cs *engine,
+			    const struct i915_request *rq,
+			    int prio)
 {
-	const struct i915_request *inflight =
-		execlists_active(&engine->execlists);
+	const struct i915_request *inflight;
+
+	/*
+	 * We only need to kick the tasklet once for the high priority
+	 * new context we add into the queue.
+	 */
+	if (prio <= engine->execlists.queue_priority_hint)
+		return;
+
+	/* Nothing currently active? We're overdue for a submission! */
+	inflight = execlists_active(&engine->execlists);
+	if (!inflight)
+		return;
 
 	/*
 	 * If we are already the currently executing context, don't
-	 * bother evaluating if we should preempt ourselves, or if
-	 * we expect nothing to change as a result of running the
-	 * tasklet, i.e. we have not change the priority queue
-	 * sufficiently to oust the running context.
+	 * bother evaluating if we should preempt ourselves.
 	 */
-	if (!inflight || !need_preempt(prio, rq_prio(inflight)))
+	if (inflight->hw_context == rq->hw_context)
 		return;
 
-	tasklet_hi_schedule(&engine->execlists.tasklet);
+	engine->execlists.queue_priority_hint = prio;
+	if (need_preempt(prio, rq_prio(inflight)))
+		tasklet_hi_schedule(&engine->execlists.tasklet);
 }
 
 static void __i915_schedule(struct i915_sched_node *node,
@@ -330,13 +342,8 @@ static void __i915_schedule(struct i915_sched_node *node,
 			list_move_tail(&node->link, cache.priolist);
 		}
 
-		if (prio <= engine->execlists.queue_priority_hint)
-			continue;
-
-		engine->execlists.queue_priority_hint = prio;
-
 		/* Defer (tasklet) submission until after all of our updates. */
-		kick_submission(engine, prio);
+		kick_submission(engine, node_to_request(node), prio);
 	}
 
 	spin_unlock(&engine->active.lock);