[RFC PATCH 6/6] sched/dl: Try not to select a too fast core

[Luca Abeni]

From: luca abeni <luca.abeni@xxxxxxxxxxxxxxx>

When a task can fit on multiple CPU cores, try to select the slowest
core that is able to properly serve the task. This avoids useless
future migrations, leaving the "fast cores" idle for more heavyweight
tasks.

Signed-off-by: luca abeni <luca.abeni@xxxxxxxxxxxxxxx>
---
 kernel/sched/cpudeadline.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 2a4ac7b529b7..897ed71af515 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -143,17 +143,24 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
 	       struct cpumask *later_mask)
 {
 	const struct sched_dl_entity *dl_se = &p->dl;
+	struct cpumask tmp_mask;
 
 	if (later_mask &&
-	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
+	    cpumask_and(&tmp_mask, cp->free_cpus, &p->cpus_allowed)) {
 		int cpu, max_cpu = -1;
-		u64 max_cap = 0;
+		u64 max_cap = 0, min_cap = SCHED_CAPACITY_SCALE * SCHED_CAPACITY_SCALE;
 
-		for_each_cpu(cpu, later_mask) {
+		cpumask_clear(later_mask);
+		for_each_cpu(cpu, &tmp_mask) {
 			u64 cap;
 
-			if (!dl_task_fit(&p->dl, cpu, &cap))
-				cpumask_clear_cpu(cpu, later_mask);
+			if (dl_task_fit(&p->dl, cpu, &cap) && (cap <= min_cap)) {
+				if (cap < min_cap) {
+					min_cap = cap;
+					cpumask_clear(later_mask);
+				}
+				cpumask_set_cpu(cpu, later_mask);
+			}
 
 			if (cap > max_cap) {
 				max_cap = cap;
-- 
2.20.1