[RFC PATCH v1 01/11] sched/idle: create a fast path for short idle periods

[Aubrey Li <aubrey.li@intel.com>]

From: Aubrey Li <aubrey.li@linux.intel.com>

Short idle periods occur common under some workloads, and the idle
entry and exit path starts to dominate, so it's important to optimize
them. A fast idle routine is introduced here for short idle periods.
- tick nohz enter/exit are exclued
- RCU idle enter/exit are excluded since tick won't be stopped
- idle governor will be leveraged, hardware idle state selection
  on some arches is excluded
- default system idle is used
- any deferrable work will be excluded
---
 kernel/sched/idle.c | 59 ++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 49 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index ac6d517..cf6c11f 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -202,22 +202,39 @@ static void cpuidle_idle_call(void)
 }
 
 /*
- * Generic idle loop implementation
- *
- * Called with polling cleared.
+ * fast idle loop implementation
  */
-static void do_idle(void)
+static void cpuidle_fast(void)
 {
+	while (!need_resched()) {
+		check_pgt_cache();
+		rmb();
+
+		if (cpu_is_offline(smp_processor_id())) {
+			cpuhp_report_idle_dead();
+			arch_cpu_idle_dead();
+		}
+
+		local_irq_disable();
+		arch_cpu_idle_enter();
+
+		default_idle_call();
+
+		arch_cpu_idle_exit();
+	}
+
 	/*
-	 * If the arch has a polling bit, we maintain an invariant:
+	 * Since we fell out of the loop above, we know TIF_NEED_RESCHED must
+	 * be set, propagate it into PREEMPT_NEED_RESCHED.
 	 *
-	 * Our polling bit is clear if we're not scheduled (i.e. if rq->curr !=
-	 * rq->idle). This means that, if rq->idle has the polling bit set,
-	 * then setting need_resched is guaranteed to cause the CPU to
-	 * reschedule.
+	 * This is required because for polling idle loops we will not have had
+	 * an IPI to fold the state for us.
 	 */
+	preempt_set_need_resched();
+}
 
-	__current_set_polling();
+static void cpuidle_generic(void)
+{
 	tick_nohz_idle_enter();
 
 	while (!need_resched()) {
@@ -254,6 +271,28 @@ static void do_idle(void)
 	 */
 	preempt_set_need_resched();
 	tick_nohz_idle_exit();
+}
+
+/*
+ * Generic idle loop implementation
+ *
+ * Called with polling cleared.
+ */
+static void do_idle(void)
+{
+	/*
+	 * If the arch has a polling bit, we maintain an invariant:
+	 *
+	 * Our polling bit is clear if we're not scheduled (i.e. if rq->curr !=
+	 * rq->idle). This means that, if rq->idle has the polling bit set,
+	 * then setting need_resched is guaranteed to cause the CPU to
+	 * reschedule.
+	 */
+
+	__current_set_polling();
+
+	cpuidle_generic();
+
 	__current_clr_polling();
 
 	/*
-- 
2.7.4