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

Aubrey Li
2017-07-10T01:48:55Z

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