[PATCH v2] sched/deadline: Make find_later_rq() choose a closer cpu in topology

[Byungchul Park]

When cpudl_find() returns any among free_cpus, the cpu might not be
closer than others, considering sched domain. For example:

   this_cpu: 15
   free_cpus: 0, 1,..., 14 (== later_mask)
   best_cpu: 0

   topology:

   0 --+
       +--+
   1 --+  |
          +-- ... --+
   2 --+  |         |
       +--+         |
   3 --+            |

   ...             ...

   12 --+           |
        +--+        |
   13 --+  |        |
           +-- ... -+
   14 --+  |
        +--+
   15 --+

In this case, it would be best to select 14 since it's a free cpu and
closest to 15(this_cpu). However, currently the code select 0(best_cpu)
even though that's just any among free_cpus. Fix it.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/sched/deadline.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a2ce590..49c93b9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1324,7 +1324,7 @@ static int find_later_rq(struct task_struct *task)
 	struct sched_domain *sd;
 	struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
 	int this_cpu = smp_processor_id();
-	int best_cpu, cpu = task_cpu(task);
+	int cpu = task_cpu(task);
 
 	/* Make sure the mask is initialized first */
 	if (unlikely(!later_mask))
@@ -1337,17 +1337,14 @@ static int find_later_rq(struct task_struct *task)
 	 * We have to consider system topology and task affinity
 	 * first, then we can look for a suitable cpu.
 	 */
-	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
-			task, later_mask);
-	if (best_cpu == -1)
+	if (cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask) == -1)
 		return -1;
 
 	/*
-	 * If we are here, some target has been found,
-	 * the most suitable of which is cached in best_cpu.
-	 * This is, among the runqueues where the current tasks
-	 * have later deadlines than the task's one, the rq
-	 * with the latest possible one.
+	 * If we are here, some targets have been found, including
+	 * the most suitable which is, among the runqueues where the
+	 * current tasks have later deadlines than the task's one, the
+	 * rq with the latest possible one.
 	 *
 	 * Now we check how well this matches with task's
 	 * affinity and system topology.
@@ -1367,6 +1364,7 @@ static int find_later_rq(struct task_struct *task)
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
 		if (sd->flags & SD_WAKE_AFFINE) {
+			int closest_cpu;
 
 			/*
 			 * If possible, preempting this_cpu is
@@ -1378,14 +1376,17 @@ static int find_later_rq(struct task_struct *task)
 				return this_cpu;
 			}
 
+			closest_cpu = cpumask_first_and(later_mask,
+							sched_domain_span(sd));
 			/*
-			 * Last chance: if best_cpu is valid and is
-			 * in the mask, that becomes our choice.
+			 * Last chance: if a cpu being in both later_mask
+			 * and current sd span is valid, that becomes our
+			 * choice. Of course, the latest possible cpu is
+			 * already under consideration through later_mask.
 			 */
-			if (best_cpu < nr_cpu_ids &&
-			    cpumask_test_cpu(best_cpu, sched_domain_span(sd))) {
+			if (closest_cpu < nr_cpu_ids) {
 				rcu_read_unlock();
-				return best_cpu;
+				return closest_cpu;
 			}
 		}
 	}
-- 
1.9.1