Avoid needlessly costly migrations. CONFIG_SCHED_CPU_AFFINITY

Given a choice between several available CPUs (unlinked) on which
to schedule a task, let the scheduler select the CPU closest to
where that task was previously scheduled.  Hopefully, this will
reduce cache migration penalties.

Notes: SCHED_CPU_AFFINITY is dependent upon x86 (only x86 is
supported at this time). Also PFair/PD^2 does not make use of
this feature.

Signed-off-by: Andrea Bastoni <bastoni@cs.unc.edu>

1 files changed, 33 insertions, 0 deletions

diff --git a/litmus/sched_cedf.c b/litmus/sched_cedf.c
index 73fe1c442a0d..e29a9fe2a8e8 100644
--- a/litmus/sched_cedf.c
+++ b/litmus/sched_cedf.c
@@ -43,6 +43,10 @@
 
 #include <litmus/bheap.h>
 
+#ifdef CONFIG_SCHED_CPU_AFFINITY
+#include <litmus/affinity.h>
+#endif
+
 /* to configure the cluster size */
 #include <litmus/litmus_proc.h>
 #include <linux/uaccess.h>
@@ -257,6 +261,23 @@ static noinline void requeue(struct task_struct* task)
         }
 }
 
+#ifdef CONFIG_SCHED_CPU_AFFINITY
+static cpu_entry_t* cedf_get_nearest_available_cpu(
+                                cedf_domain_t *cluster, cpu_entry_t* start)
+{
+        cpu_entry_t* affinity;
+
+        get_nearest_available_cpu(affinity, start, cedf_cpu_entries, -1);
+
+        /* make sure CPU is in our cluster */
+        if (affinity && cpu_isset(affinity->cpu, *cluster->cpu_map))
+                return(affinity);
+        else
+                return(NULL);
+}
+#endif
+
+
 /* check for any necessary preemptions */
 static void check_for_preemptions(cedf_domain_t *cluster)
 {
@@ -270,8 +291,20 @@ static void check_for_preemptions(cedf_domain_t *cluster)
                 task = __take_ready(&cluster->domain);
                 TRACE("check_for_preemptions: attempting to link task %d to %d\n",
                       task->pid, last->cpu);
+#ifdef CONFIG_SCHED_CPU_AFFINITY
+                {
+                        cpu_entry_t* affinity =
+                                        cedf_get_nearest_available_cpu(cluster,
+                                                &per_cpu(cedf_cpu_entries, task_cpu(task)));
+                        if(affinity)
+                                last = affinity;
+                        else if(last->linked)
+                                requeue(last->linked);
+                }
+#else
                 if (last->linked)
                         requeue(last->linked);
+#endif
                 link_task_to_cpu(task, last);
                 preempt(last);
         }