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.p...@lge.com>
---
 kernel/sched/rt.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 979b734..50639e5 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1618,12 +1618,35 @@ static struct task_struct 
*pick_highest_pushable_task(struct rq *rq, int cpu)
 
 static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);
 
+/*
+ * Find the first cpu in: mask & sd & ~prefer
+ */
+static int find_cpu(const struct cpumask *mask,
+                   const struct sched_domain *sd,
+                   const struct sched_domain *prefer)
+{
+       const struct cpumask *sds = sched_domain_span(sd);
+       const struct cpumask *ps  = prefer ? sched_domain_span(prefer) : NULL;
+       int cpu = -1;
+
+       while ((cpu = cpumask_next(cpu, mask)) < nr_cpu_ids) {
+               if (!cpumask_test_cpu(cpu, sds))
+                       continue;
+               if (ps && cpumask_test_cpu(cpu, ps))
+                       continue;
+               break;
+       }
+
+       return cpu;
+}
+
 static int find_lowest_rq(struct task_struct *task)
 {
-       struct sched_domain *sd;
+       struct sched_domain *sd, *prefer = NULL;
        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))
@@ -1668,9 +1691,20 @@ static int find_lowest_rq(struct task_struct *task)
                                return this_cpu;
                        }
 
-                       best_cpu = cpumask_first_and(lowest_mask,
-                                                    sched_domain_span(sd));
+                       best_cpu = find_cpu(lowest_mask, sd, prefer);
+
                        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) {
+                                       prefer = sd;
+                                       if (fallback_cpu == -1)
+                                               fallback_cpu = best_cpu;
+                                       continue;
+                               }
                                rcu_read_unlock();
                                return best_cpu;
                        }
@@ -1679,6 +1713,13 @@ 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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