[RFC PATCH v2 2/8] cpuidle: record the overhead of idle entry

[Aubrey Li]

Record the overhead of idle entry in micro-second

Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
---
 drivers/cpuidle/cpuidle.c | 33 +++++++++++++++++++++++++++++++++
 include/linux/cpuidle.h   | 14 ++++++++++++++
 kernel/sched/idle.c       |  8 +++++++-
 3 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 60bb64f..4066308 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -302,6 +302,39 @@ void cpuidle_reflect(struct cpuidle_device *dev, int index)
 		cpuidle_curr_governor->reflect(dev, index);
 }
 
+/* cpuidle_entry_start - record idle entry start */
+void cpuidle_entry_start(void)
+{
+	struct cpuidle_device *dev = cpuidle_get_device();
+
+	if (dev)
+		dev->idle_stat.entry_start = local_clock();
+}
+
+/*
+ * cpuidle_entry_end - record idle entry end, and maintain
+ * the entry overhead average in micro-second
+ */
+void cpuidle_entry_end(void)
+{
+	struct cpuidle_device *dev = cpuidle_get_device();
+	u64 overhead;
+	s64 diff;
+
+	if (dev) {
+		dev->idle_stat.entry_end = local_clock();
+		overhead = div_u64(dev->idle_stat.entry_end -
+				dev->idle_stat.entry_start, NSEC_PER_USEC);
+		diff = overhead - dev->idle_stat.overhead;
+		dev->idle_stat.overhead += diff >> 3;
+		/*
+		 * limit overhead to 1us
+		 */
+		if (dev->idle_stat.overhead == 0)
+			dev->idle_stat.overhead = 1;
+	}
+}
+
 /**
  * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
  */
diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
index fc1e5d7..cad9b71 100644
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -72,6 +72,15 @@ struct cpuidle_device_kobj;
 struct cpuidle_state_kobj;
 struct cpuidle_driver_kobj;
 
+struct cpuidle_stat {
+	u64			entry_start;	/* nanosecond */
+	u64			entry_end;	/* nanosecond */
+	u64			overhead;	/* nanosecond */
+	unsigned int		predicted_us;	/* microsecond */
+	bool			predicted;	/* ever predicted? */
+	bool			fast_idle;	/* fast idle? */
+};
+
 struct cpuidle_device {
 	unsigned int		registered:1;
 	unsigned int		enabled:1;
@@ -89,6 +98,7 @@ struct cpuidle_device {
 	cpumask_t		coupled_cpus;
 	struct cpuidle_coupled	*coupled;
 #endif
+	struct cpuidle_stat	idle_stat;
 };
 
 DECLARE_PER_CPU(struct cpuidle_device *, cpuidle_devices);
@@ -131,6 +141,8 @@ extern bool cpuidle_not_available(struct cpuidle_driver *drv,
 
 extern int cpuidle_select(struct cpuidle_driver *drv,
 			  struct cpuidle_device *dev);
+extern void cpuidle_entry_start(void);
+extern void cpuidle_entry_end(void);
 extern int cpuidle_enter(struct cpuidle_driver *drv,
 			 struct cpuidle_device *dev, int index);
 extern void cpuidle_reflect(struct cpuidle_device *dev, int index);
@@ -164,6 +176,8 @@ static inline bool cpuidle_not_available(struct cpuidle_driver *drv,
 static inline int cpuidle_select(struct cpuidle_driver *drv,
 				 struct cpuidle_device *dev)
 {return -ENODEV; }
+static inline void cpuidle_entry_start(void) { }
+static inline void cpuidle_entry_end(void) { }
 static inline int cpuidle_enter(struct cpuidle_driver *drv,
 				struct cpuidle_device *dev, int index)
 {return -ENODEV; }
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 6c23e30..0951dac 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -210,6 +210,12 @@ static void cpuidle_idle_call(void)
 static void do_idle(void)
 {
 	/*
+	 * we record idle entry overhead now, so any deferrable items
+	 * in idle entry path need to be placed between cpuidle_entry_start()
+	 * and cpuidle_entry_end()
+	 */
+	cpuidle_entry_start();
+	/*
 	 * 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 !=
@@ -217,10 +223,10 @@ static void do_idle(void)
 	 * then setting need_resched is guaranteed to cause the CPU to
 	 * reschedule.
 	 */
-
 	__current_set_polling();
 	quiet_vmstat();
 	tick_nohz_idle_enter();
+	cpuidle_entry_end();
 
 	while (!need_resched()) {
 		check_pgt_cache();
-- 
2.7.4