GSN-EDF: link tasks to local CPU if it is idle

If a task is released on a CPU that is idle, then it is pointless to
send an IPI to another CPU just to schedule the newly released task.
This patch changes check_for_preemptions() to prefer the local CPU
first if it is idle.

1 files changed, 12 insertions, 0 deletions

diff --git a/litmus/sched_gsn_edf.c b/litmus/sched_gsn_edf.c
index 5a63ef1c580f..3bf484c3f857 100644
--- a/litmus/sched_gsn_edf.c
+++ b/litmus/sched_gsn_edf.c
@@ -280,6 +280,18 @@ static void check_for_preemptions(void)
 {
         struct task_struct *task;
         cpu_entry_t *last;
+        cpu_entry_t *local;
+
+        /* Before linking to other CPUs, check first whether the local CPU is
+         * idle. */
+        local = &__get_cpu_var(gsnedf_cpu_entries);
+        task  = __peek_ready(&gsnedf);
+        if (task && !local->linked) {
+                task = __take_ready(&gsnedf);
+                TRACE_TASK(task, "linking to local CPU %d to avoid IPI\n", local->cpu);
+                link_task_to_cpu(task, local);
+                preempt(local);
+        }
 
         for (last = lowest_prio_cpu();
              edf_preemption_needed(&gsnedf, last->linked);