sched/eevdf: Fix pick_eevdf()

The old pick_eevdf() could fail to find the actual earliest eligible
deadline when it descended to the right looking for min_deadline, but
it turned out that that min_deadline wasn't actually eligible. In that
case we need to go back and search through any left branches we
skipped looking for the actual best _eligible_ min_deadline.

This is more expensive, but still O(log n), and at worst should only
involve descending two branches of the rbtree.

I've run this through a userspace stress test (thank you
tools/lib/rbtree.c), so hopefully this implementation doesn't miss any
corner cases.

Fixes: 147f3ef ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/xm261qego72d.fsf_-_@google.com

Author: Benjamin Segall

kernel/sched/fair.c

@@ -872,14 +872,16 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
  *
  * Which allows an EDF like search on (sub)trees.
  */
-static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
+static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
 {
 	struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
 	struct sched_entity *curr = cfs_rq->curr;
 	struct sched_entity *best = NULL;
+	struct sched_entity *best_left = NULL;
 
 	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
 		curr = NULL;
+	best = curr;
 
 	/*
 	 * Once selected, run a task until it either becomes non-eligible or
@@ -900,41 +902,83 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
 		}
 
 		/*
-		 * If this entity has an earlier deadline than the previous
-		 * best, take this one. If it also has the earliest deadline
-		 * of its subtree, we're done.
+		 * Now we heap search eligible trees for the best (min_)deadline
 		 */
-		if (!best || deadline_gt(deadline, best, se)) {
+		if (!best || deadline_gt(deadline, best, se))
 			best = se;
-			if (best->deadline == best->min_deadline)
-				break;
-		}
 
 		/*
-		 * If the earlest deadline in this subtree is in the fully
-		 * eligible left half of our space, go there.
+		 * Every se in a left branch is eligible, keep track of the
+		 * branch with the best min_deadline
 		 */
+		if (node->rb_left) {
+			struct sched_entity *left = __node_2_se(node->rb_left);
+
+			if (!best_left || deadline_gt(min_deadline, best_left, left))
+				best_left = left;
+
+			/*
+			 * min_deadline is in the left branch. rb_left and all
+			 * descendants are eligible, so immediately switch to the second
+			 * loop.
+			 */
+			if (left->min_deadline == se->min_deadline)
+				break;
+		}
+
+		/* min_deadline is at this node, no need to look right */
+		if (se->deadline == se->min_deadline)
+			break;
+
+		/* else min_deadline is in the right branch. */
+		node = node->rb_right;
+	}
+
+	/*
+	 * We ran into an eligible node which is itself the best.
+	 * (Or nr_running == 0 and both are NULL)
+	 */
+	if (!best_left || (s64)(best_left->min_deadline - best->deadline) > 0)
+		return best;
+
+	/*
+	 * Now best_left and all of its children are eligible, and we are just
+	 * looking for deadline == min_deadline
+	 */
+	node = &best_left->run_node;
+	while (node) {
+		struct sched_entity *se = __node_2_se(node);
+
+		/* min_deadline is the current node */
+		if (se->deadline == se->min_deadline)
+			return se;
+
+		/* min_deadline is in the left branch */
 		if (node->rb_left &&
 		    __node_2_se(node->rb_left)->min_deadline == se->min_deadline) {
 			node = node->rb_left;
 			continue;
 		}
 
+		/* else min_deadline is in the right branch */
 		node = node->rb_right;
 	}
+	return NULL;
+}
 
-	if (!best || (curr && deadline_gt(deadline, best, curr)))
-		best = curr;
+static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
+{
+	struct sched_entity *se = __pick_eevdf(cfs_rq);
 
-	if (unlikely(!best)) {
+	if (!se) {
 		struct sched_entity *left = __pick_first_entity(cfs_rq);
 		if (left) {
 			pr_err("EEVDF scheduling fail, picking leftmost\n");
 			return left;
 		}
 	}
 
-	return best;
+	return se;
 }
 
 #ifdef CONFIG_SCHED_DEBUG