sched: Do not use IPIs for remote wakeups if idle

Do not wake up and idle CPU for the chance of a cache hit. This avoids
unnecessary energy consumption. Plus, if we need to wake an idle CPU to
handle a remote wakeup, we will induce additional scheduling latency
since the remote CPU will be unavailable while leaving an idle state.

Signed-off-by: Tyler Nijmeh <tylernij@gmail.com>

Author: tytydraco

kernel/sched/core.c

@@ -1957,8 +1957,10 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
 	struct rq_flags rf;
 
 #if defined(CONFIG_SMP)
-	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
-		sched_clock_cpu(cpu); /* sync clocks x-cpu */
+	if (sched_feat(TTWU_QUEUE) &&
+			!idle_cpu(cpu) &&
+			!cpus_share_cache(smp_processor_id(), cpu)) {
+		sched_clock_cpu(cpu); /* Sync clocks across CPUs */
 		ttwu_queue_remote(p, cpu, wake_flags);
 		return;
 	}