drm/i915: Rename execlists->queue_priority to queue_priority_hint

After noticing that we trigger preemption events for currently executing
requests, as well as requests that complete before the preemption and
attempting to suppress those preemption events, it is wise to not
consider the queue_priority to be authoritative. As we only track the
maximum priority seen between dequeue passes, if the maximum priority
request is no longer available for dequeuing (it completed or is even
executing on another engine), we have no knowledge of the previous
queue_priority as it would require us to keep a full history of enqueued
requests -- but we already have that history in the priolists!

Rename the queue_priority to queue_priority_hint so that we do not
confuse it as being exactly the maximum priority in the queue, but merely
an indication that we have seen a new maximum priority value and as such
we should check whether it should preempt the currently running request.

v2: s/preempt_priority_hint/queue_priority_hint/ as preempt implies it
being only used for the singular task of preemption and not the wider
question of waking up due to a change in the queue.
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/20190129185452.20989-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_scheduler.c

@@ -127,8 +127,7 @@ static inline struct i915_priolist *to_priolist(struct rb_node *rb)
 	return rb_entry(rb, struct i915_priolist, node);
 }
 
-static void assert_priolists(struct intel_engine_execlists * const execlists,
-			     long queue_priority)
+static void assert_priolists(struct intel_engine_execlists * const execlists)
 {
 	struct rb_node *rb;
 	long last_prio, i;
@@ -139,7 +138,7 @@ static void assert_priolists(struct intel_engine_execlists * const execlists,
 	GEM_BUG_ON(rb_first_cached(&execlists->queue) !=
 		   rb_first(&execlists->queue.rb_root));
 
-	last_prio = (queue_priority >> I915_USER_PRIORITY_SHIFT) + 1;
+	last_prio = (INT_MAX >> I915_USER_PRIORITY_SHIFT) + 1;
 	for (rb = rb_first_cached(&execlists->queue); rb; rb = rb_next(rb)) {
 		const struct i915_priolist *p = to_priolist(rb);
 
@@ -166,7 +165,7 @@ i915_sched_lookup_priolist(struct intel_engine_cs *engine, int prio)
 	int idx, i;
 
 	lockdep_assert_held(&engine->timeline.lock);
-	assert_priolists(execlists, INT_MAX);
+	assert_priolists(execlists);
 
 	/* buckets sorted from highest [in slot 0] to lowest priority */
 	idx = I915_PRIORITY_COUNT - (prio & I915_PRIORITY_MASK) - 1;
@@ -353,7 +352,7 @@ static void __i915_schedule(struct i915_request *rq,
 				continue;
 		}
 
-		if (prio <= engine->execlists.queue_priority)
+		if (prio <= engine->execlists.queue_priority_hint)
 			continue;
 
 		/*
@@ -366,7 +365,7 @@ static void __i915_schedule(struct i915_request *rq,
 			continue;
 
 		/* Defer (tasklet) submission until after all of our updates. */
-		engine->execlists.queue_priority = prio;
+		engine->execlists.queue_priority_hint = prio;
 		tasklet_hi_schedule(&engine->execlists.tasklet);
 	}
 

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -480,7 +480,7 @@ static void intel_engine_init_execlist(struct intel_engine_cs *engine)
 	GEM_BUG_ON(!is_power_of_2(execlists_num_ports(execlists)));
 	GEM_BUG_ON(execlists_num_ports(execlists) > EXECLIST_MAX_PORTS);
 
-	execlists->queue_priority = INT_MIN;
+	execlists->queue_priority_hint = INT_MIN;
 	execlists->queue = RB_ROOT_CACHED;
 }
 
@@ -1178,7 +1178,7 @@ void intel_engines_park(struct drm_i915_private *i915)
 		}
 
 		/* Must be reset upon idling, or we may miss the busy wakeup. */
-		GEM_BUG_ON(engine->execlists.queue_priority != INT_MIN);
+		GEM_BUG_ON(engine->execlists.queue_priority_hint != INT_MIN);
 
 		if (engine->park)
 			engine->park(engine);

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -737,7 +737,7 @@ static bool __guc_dequeue(struct intel_engine_cs *engine)
 		if (intel_engine_has_preemption(engine)) {
 			struct guc_preempt_work *preempt_work =
 				&engine->i915->guc.preempt_work[engine->id];
-			int prio = execlists->queue_priority;
+			int prio = execlists->queue_priority_hint;
 
 			if (__execlists_need_preempt(prio, port_prio(port))) {
 				execlists_set_active(execlists,
@@ -783,7 +783,8 @@ static bool __guc_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_hint =
+		rb ? to_priolist(rb)->priority : INT_MIN;
 	if (submit)
 		port_assign(port, last);
 	if (last)

drivers/gpu/drm/i915/intel_lrc.c

@@ -591,7 +591,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
 			return;
 
-		if (need_preempt(engine, last, execlists->queue_priority)) {
+		if (need_preempt(engine, last, execlists->queue_priority_hint)) {
 			inject_preempt_context(engine);
 			return;
 		}
@@ -699,20 +699,20 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	/*
 	 * Here be a bit of magic! Or sleight-of-hand, whichever you prefer.
 	 *
-	 * We choose queue_priority such that if we add a request of greater
+	 * We choose the priority hint such that if we add a request of greater
 	 * priority than this, we kick the submission tasklet to decide on
 	 * the right order of submitting the requests to hardware. We must
 	 * also be prepared to reorder requests as they are in-flight on the
-	 * HW. We derive the queue_priority then as the first "hole" in
+	 * HW. We derive the priority hint then as the first "hole" in
 	 * the HW submission ports and if there are no available slots,
 	 * the priority of the lowest executing request, i.e. last.
 	 *
 	 * When we do receive a higher priority request ready to run from the
-	 * user, see queue_request(), the queue_priority is bumped to that
+	 * user, see queue_request(), the priority hint is bumped to that
 	 * request triggering preemption on the next dequeue (or subsequent
 	 * interrupt for secondary ports).
 	 */
-	execlists->queue_priority =
+	execlists->queue_priority_hint =
 		port != execlists->port ? rq_prio(last) : INT_MIN;
 
 	if (submit) {
@@ -861,7 +861,7 @@ static void execlists_cancel_requests(struct intel_engine_cs *engine)
 
 	/* Remaining _unready_ requests will be nop'ed when submitted */
 
-	execlists->queue_priority = INT_MIN;
+	execlists->queue_priority_hint = INT_MIN;
 	execlists->queue = RB_ROOT_CACHED;
 	GEM_BUG_ON(port_isset(execlists->port));
 
@@ -1092,8 +1092,8 @@ static void __submit_queue_imm(struct intel_engine_cs *engine)
 
 static void submit_queue(struct intel_engine_cs *engine, int prio)
 {
-	if (prio > engine->execlists.queue_priority) {
-		engine->execlists.queue_priority = prio;
+	if (prio > engine->execlists.queue_priority_hint) {
+		engine->execlists.queue_priority_hint = prio;
 		__submit_queue_imm(engine);
 	}
 }
@@ -2777,7 +2777,9 @@ void intel_execlists_show_requests(struct intel_engine_cs *engine,
 
 	last = NULL;
 	count = 0;
-	drm_printf(m, "\t\tQueue priority: %d\n", execlists->queue_priority);
+	if (execlists->queue_priority_hint != INT_MIN)
+		drm_printf(m, "\t\tQueue priority hint: %d\n",
+			   execlists->queue_priority_hint);
 	for (rb = rb_first_cached(&execlists->queue); rb; rb = rb_next(rb)) {
 		struct i915_priolist *p = rb_entry(rb, typeof(*p), node);
 		int i;

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -293,14 +293,18 @@ struct intel_engine_execlists {
 	unsigned int port_mask;
 
 	/**
-	 * @queue_priority: Highest pending priority.
+	 * @queue_priority_hint: Highest pending priority.
 	 *
 	 * When we add requests into the queue, or adjust the priority of
 	 * executing requests, we compute the maximum priority of those
 	 * pending requests. We can then use this value to determine if
 	 * we need to preempt the executing requests to service the queue.
+	 * However, since the we may have recorded the priority of an inflight
+	 * request we wanted to preempt but since completed, at the time of
+	 * dequeuing the priority hint may no longer may match the highest
+	 * available request priority.
 	 */
-	int queue_priority;
+	int queue_priority_hint;
 
 	/**
 	 * @queue: queue of requests, in priority lists