[PATCH 62/62] drm/i915: Improve DFS for priority inheritance

[Chris Wilson]

The core of the scheduling algorithm is that we compute the topological
order of the fence DAG. Knowing that we have a DAG, we should be able to
use a DFS to compute the topological sort in linear time. However,
during the conversion of the recursive algorithm into an iterative one,
the memorization of how far we had progressed down a branch was
forgotten.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_scheduler.c | 61 ++++++++++++++++++---------
 1 file changed, 41 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 98816a9f147d..95adec09674a 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -251,15 +251,38 @@ static void ipi_priority(struct i915_dependency *p, int prio)
 	spin_unlock(&ipi_lock);
 }
 
+static struct i915_request *
+stack_push(struct i915_request *rq, struct i915_request *stack)
+{
+	rq->sched.dfs.prev = &rq->sched.signalers_list;
+	rq->sched.dfs.next = (struct list_head *)stack;
+	return rq;
+}
+
+static void stack_store(struct i915_request *rq, struct i915_dependency *p)
+{
+	rq->sched.dfs.prev = &p->signal_link;
+}
+
+static struct i915_dependency *stack_load(struct i915_request *rq)
+{
+	return list_entry_rcu(rq->sched.dfs.prev->next,
+			      struct i915_dependency,
+			      signal_link);
+}
+
+static struct i915_request *stack_pop(struct i915_request *rq)
+{
+	return (struct i915_request *)rq->sched.dfs.next;
+}
+
 static void __i915_request_set_priority(struct i915_request *rq, int prio)
 {
 	struct intel_engine_cs *engine = rq->engine;
-	struct i915_request *rn;
 	struct list_head *plist;
-	LIST_HEAD(dfs);
 
 	lockdep_assert_held(&engine->active.lock);
-	list_add(&rq->sched.dfs, &dfs);
+	plist = i915_sched_lookup_priolist(engine, prio);
 
 	/*
 	 * Recursively bump all dependent priorities to match the new request.
@@ -279,13 +302,15 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
 	 * end result is a topological list of requests in reverse order, the
 	 * last element in the list is the request we must execute first.
 	 */
-	list_for_each_entry(rq, &dfs, sched.dfs) {
+	stack_push(rq, NULL);
+	do {
 		struct i915_dependency *p;
 
-		/* Also release any children on this engine that are ready */
-		GEM_BUG_ON(rq->engine != engine);
-
-		for_each_signaler(p, rq) {
+descend:
+		p = stack_load(rq);
+		list_for_each_entry_from_rcu(p,
+					     &rq->sched.signalers_list,
+					     signal_link) {
 			struct i915_request *s =
 				container_of(p->signaler, typeof(*s), sched);
 
@@ -297,22 +322,18 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
 			if (i915_request_completed(s))
 				continue;
 
-			if (s->engine != rq->engine) {
+			if (s->engine != engine) {
 				ipi_priority(p, prio);
 				continue;
 			}
 
-			list_move_tail(&s->sched.dfs, &dfs);
+			/* Remember where we our position along this branch */
+			stack_store(rq, p);
+			rq = stack_push(s, rq);
+			goto descend;
 		}
-	}
 
-	plist = i915_sched_lookup_priolist(engine, prio);
-
-	/* Fifo and depth-first replacement ensure our deps execute first */
-	list_for_each_entry_safe_reverse(rq, rn, &dfs, sched.dfs) {
-		GEM_BUG_ON(rq->engine != engine);
-
-		INIT_LIST_HEAD(&rq->sched.dfs);
+		RQ_TRACE(rq, "set-priority:%d\n", prio);
 		WRITE_ONCE(rq->sched.attr.priority, prio);
 
 		/*
@@ -326,12 +347,13 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
 		if (!i915_request_is_ready(rq))
 			continue;
 
+		GEM_BUG_ON(rq->engine != engine);
 		if (i915_request_in_priority_queue(rq))
 			list_move_tail(&rq->sched.link, plist);
 
 		/* Defer (tasklet) submission until after all updates. */
 		kick_submission(engine, rq, prio);
-	}
+	} while ((rq = stack_pop(rq)));
 }
 
 void i915_request_set_priority(struct i915_request *rq, int prio)
@@ -398,7 +420,6 @@ void i915_sched_node_init(struct i915_sched_node *node)
 	INIT_LIST_HEAD(&node->signalers_list);
 	INIT_LIST_HEAD(&node->waiters_list);
 	INIT_LIST_HEAD(&node->link);
-	INIT_LIST_HEAD(&node->dfs);
 
 	i915_sched_node_reinit(node);
 }
-- 
2.20.1