drm/i915/preemption: Allow preemption between submission ports

Sometimes we need to boost the priority of an in-flight request, which
may lead to the situation where the second submission port then contains
a higher priority context than the first and so we need to inject a
preemption event. To do so we must always check inside
execlists_dequeue() whether there is a priority inversion between the
ports themselves as well as the head of the priority sorted queue, and we
cannot just skip dequeuing if the queue is empty.

As Michał noted, this doesn't simply extend to handling more than 2-port
submission, as we may need to reorder within the array of executing
requests which themselves are lower priority than the first. A task for
later!
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222142229.14517-1-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -423,6 +423,7 @@ static void intel_engine_init_execlist(struct intel_engine_cs *engine)
 	BUILD_BUG_ON_NOT_POWER_OF_2(execlists_num_ports(execlists));
 	GEM_BUG_ON(execlists_num_ports(execlists) > EXECLIST_MAX_PORTS);
 
+	execlists->queue_priority = INT_MIN;
 	execlists->queue = RB_ROOT;
 	execlists->first = NULL;
 }
@@ -1903,6 +1904,7 @@ void intel_engine_dump(struct intel_engine_cs *engine,
 	spin_lock_irq(&engine->timeline->lock);
 	list_for_each_entry(rq, &engine->timeline->requests, link)
 		print_request(m, rq, "\t\tE ");
+	drm_printf(m, "\t\tQueue priority: %d\n", execlists->queue_priority);
 	for (rb = execlists->first; rb; rb = rb_next(rb)) {
 		struct i915_priolist *p =
 			rb_entry(rb, typeof(*p), node);

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -75,6 +75,11 @@
  *
  */
 
+static inline struct i915_priolist *to_priolist(struct rb_node *rb)
+{
+	return rb_entry(rb, struct i915_priolist, node);
+}
+
 static inline bool is_high_priority(struct intel_guc_client *client)
 {
 	return (client->priority == GUC_CLIENT_PRIORITY_KMD_HIGH ||
@@ -682,15 +687,12 @@ static void guc_dequeue(struct intel_engine_cs *engine)
 	rb = execlists->first;
 	GEM_BUG_ON(rb_first(&execlists->queue) != rb);
 
-	if (!rb)
-		goto unlock;
-
 	if (port_isset(port)) {
 		if (engine->i915->preempt_context) {
 			struct guc_preempt_work *preempt_work =
 				&engine->i915->guc.preempt_work[engine->id];
 
-			if (rb_entry(rb, struct i915_priolist, node)->priority >
+			if (execlists->queue_priority >
 			    max(port_request(port)->priotree.priority, 0)) {
 				execlists_set_active(execlists,
 						     EXECLISTS_ACTIVE_PREEMPT);
@@ -706,8 +708,8 @@ static void guc_dequeue(struct intel_engine_cs *engine)
 	}
 	GEM_BUG_ON(port_isset(port));
 
-	do {
-		struct i915_priolist *p = rb_entry(rb, typeof(*p), node);
+	while (rb) {
+		struct i915_priolist *p = to_priolist(rb);
 		struct i915_request *rq, *rn;
 
 		list_for_each_entry_safe(rq, rn, &p->requests, priotree.link) {
@@ -736,8 +738,9 @@ static void guc_dequeue(struct intel_engine_cs *engine)
 		INIT_LIST_HEAD(&p->requests);
 		if (p->priority != I915_PRIORITY_NORMAL)
 			kmem_cache_free(engine->i915->priorities, p);
-	} while (rb);
+	}
 done:
+	execlists->queue_priority = rb ? to_priolist(rb)->priority : INT_MIN;
 	execlists->first = rb;
 	if (submit) {
 		port_assign(port, last);

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -257,6 +257,16 @@ struct intel_engine_execlists {
 	 */
 	unsigned int port_mask;
 
+	/**
+	 * @queue_priority: 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.
+	 */
+	int queue_priority;
+
 	/**
 	 * @queue: queue of requests, in priority lists
 	 */