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[PATCH v5 0/4] Make find_later_rq() choose a closer cpu in topology

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  [PATCH v5 0/4] Make find_later_rq() choose a closer cpu in topology Byungchul Park <byungchul.park@lge.com> - 2017-05-23 04:10 +0200
    [PATCH v5 4/4] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq() Byungchul Park <byungchul.park@lge.com> - 2017-05-23 04:10 +0200
    [PATCH v5 2/4] sched/deadline: Change return value of cpudl_find() Byungchul Park <byungchul.park@lge.com> - 2017-05-23 04:10 +0200

#1647574 — [PATCH v5 0/4] Make find_later_rq() choose a closer cpu in topology

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.

Change from v4
   -. remove a patch that might cause huge lock contention
      (by spin lock(&cpudl.lock) in a hot path of scheduler)

Change from v3
   -. rename closest_cpu to best_cpu so that it align with rt
   -. protect referring cpudl.elements with cpudl.lock
   -. change return value of cpudl_find() to bool

Change from v2
   -. add support for SD_PREFER_SIBLING

Change from v1
   -. clean up the patch

Byungchul Park (4):
  sched/deadline: Make find_later_rq() choose a closer cpu in topology
  sched/deadline: Change return value of cpudl_find()
  sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()
  sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()

 kernel/sched/cpudeadline.c | 26 ++++++++++++-------------
 kernel/sched/deadline.c    | 48 +++++++++++++++++++++++++++++++---------------
 kernel/sched/rt.c          | 17 ++++++++++++++++
 3 files changed, 63 insertions(+), 28 deletions(-)

-- 
1.9.1

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#1647575 — [PATCH v5 4/4] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()

It would be better to avoid pushing tasks to other cpu within
a SD_PREFER_SIBLING domain, instead, get more chances to check other
siblings.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 kernel/sched/rt.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 979b734..6332b2ad 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1624,6 +1624,7 @@ static int find_lowest_rq(struct task_struct *task)
 	struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
 	int this_cpu = smp_processor_id();
 	int cpu      = task_cpu(task);
+	int fallback_cpu = -1;
 
 	/* Make sure the mask is initialized first */
 	if (unlikely(!lowest_mask))
@@ -1671,6 +1672,15 @@ static int find_lowest_rq(struct task_struct *task)
 			best_cpu = cpumask_first_and(lowest_mask,
 						     sched_domain_span(sd));
 			if (best_cpu < nr_cpu_ids) {
+				/*
+				 * If current domain is SD_PREFER_SIBLING
+				 * flaged, we have to get more chances to
+				 * check other siblings.
+				 */
+				if (sd->flags & SD_PREFER_SIBLING) {
+					fallback_cpu = best_cpu;
+					continue;
+				}
 				rcu_read_unlock();
 				return best_cpu;
 			}
@@ -1679,6 +1689,13 @@ static int find_lowest_rq(struct task_struct *task)
 	rcu_read_unlock();
 
 	/*
+	 * If fallback_cpu is valid, all our quesses failed *except* for
+	 * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
+	 */
+	if (fallback_cpu != -1)
+		return fallback_cpu;
+
+	/*
 	 * And finally, if there were no matches within the domains
 	 * just give the caller *something* to work with from the compatible
 	 * locations.
-- 
1.9.1

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#1647576 — [PATCH v5 2/4] sched/deadline: Change return value of cpudl_find()

Currently cpudl_find() returns the best cpu that means it has the
maximum dl, however, the value is already kept in later_mask and the
return value is not referred directly any more.

Now, it's enough to return whether CPUs were found or not, like rt.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 kernel/sched/cpudeadline.c | 26 +++++++++++++-------------
 kernel/sched/deadline.c    |  6 +++---
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index fba235c..7408cbe 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -119,29 +119,29 @@ static inline int cpudl_maximum(struct cpudl *cp)
  * @p: the task
  * @later_mask: a mask to fill in with the selected CPUs (or NULL)
  *
- * Returns: int - best CPU (heap maximum if suitable)
+ * Returns: (int)bool - CPUs were found
  */
 int cpudl_find(struct cpudl *cp, struct task_struct *p,
 	       struct cpumask *later_mask)
 {
-	int best_cpu = -1;
 	const struct sched_dl_entity *dl_se = &p->dl;
 
 	if (later_mask &&
 	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
-		best_cpu = cpumask_any(later_mask);
-		goto out;
-	} else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
-			dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
-		best_cpu = cpudl_maximum(cp);
-		if (later_mask)
-			cpumask_set_cpu(best_cpu, later_mask);
-	}
+		return 1;
+	} else {
+		int best_cpu = cpudl_maximum(cp);
+		WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
 
-out:
-	WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
+		if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
+		    dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
+			if (later_mask)
+				cpumask_set_cpu(best_cpu, later_mask);
 
-	return best_cpu;
+			return 1;
+		}
+	}
+	return 0;
 }
 
 /*
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 9d997d9..0223694 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1107,7 +1107,7 @@ static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
 	 * let's hope p can move out.
 	 */
 	if (rq->curr->nr_cpus_allowed == 1 ||
-	    cpudl_find(&rq->rd->cpudl, rq->curr, NULL) == -1)
+	    !cpudl_find(&rq->rd->cpudl, rq->curr, NULL))
 		return;
 
 	/*
@@ -1115,7 +1115,7 @@ static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
 	 * see if it is pushed or pulled somewhere else.
 	 */
 	if (p->nr_cpus_allowed != 1 &&
-	    cpudl_find(&rq->rd->cpudl, p, NULL) != -1)
+	    cpudl_find(&rq->rd->cpudl, p, NULL))
 		return;
 
 	resched_curr(rq);
@@ -1337,7 +1337,7 @@ 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.
 	 */
-	if (cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask) == -1)
+	if (!cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask))
 		return -1;
 
 	/*
-- 
1.9.1

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