Prevent Linux to send IPI and queue tasks on remote CPUs.wip-merge-3.0

Whether to send IPIs and enqueue tasks on remote runqueues is
plugin-specific. The recent ttwu_queue() mechanism (by calling
ttwu_queue_remote()) interferes with Litmus plugin decisions.

As a side effect, note that this patch removes a synchronization
point for CPUs clocks (sched_clock_cpu()).

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 0c8b6e1e9e79..8abc7b0442c3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2666,7 +2666,12 @@ static void ttwu_queue(struct task_struct *p, int cpu)
         struct rq *rq = cpu_rq(cpu);
 
 #if defined(CONFIG_SMP)
-        if (sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) {
+        /*
+         * LITMUS^RT: whether to send an IPI to the remote CPU
+         * is plugin specific.
+         */
+        if (!is_realtime(p) &&
+                        sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) {
                 sched_clock_cpu(cpu); /* sync clocks x-cpu */
                 ttwu_queue_remote(p, cpu);
                 return;