cpuidle: menu: Skip tick_nohz_get_sleep_length() call in some cases

Because the cost of calling tick_nohz_get_sleep_length() may increase
in the future, reorder the code in menu_select() so it first uses the
statistics to determine the expected idle duration.  If that value is
higher than RESIDENCY_THRESHOLD_NS, tick_nohz_get_sleep_length() will
be called to obtain the time till the closest timer and refine the
idle duration prediction if necessary.

This causes the governor to always take the full overhead of
get_typical_interval() with the assumption that the cost will be
amortized by skipping the tick_nohz_get_sleep_length() call in the
cases when the predicted idle duration is relatively very small.

Signed-off-by: Rafael J. Wysocki <[email protected]>
Tested-by: Doug Smythies <[email protected]>

Author: rafaeljw

drivers/cpuidle/governors/gov.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/* Common definitions for cpuidle governors. */
+
+#ifndef __CPUIDLE_GOVERNOR_H
+#define __CPUIDLE_GOVERNOR_H
+
+/*
+ * Idle state target residency threshold used for deciding whether or not to
+ * check the time till the closest expected timer event.
+ */
+#define RESIDENCY_THRESHOLD_NS	(15 * NSEC_PER_USEC)
+
+#endif /* __CPUIDLE_GOVERNOR_H */

drivers/cpuidle/governors/menu.c

@@ -19,6 +19,8 @@
 #include <linux/sched/stat.h>
 #include <linux/math64.h>
 
+#include "gov.h"
+
 #define BUCKETS 12
 #define INTERVAL_SHIFT 3
 #define INTERVALS (1UL << INTERVAL_SHIFT)
@@ -166,8 +168,7 @@ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
  * of points is below a threshold. If it is... then use the
  * average of these 8 points as the estimated value.
  */
-static unsigned int get_typical_interval(struct menu_device *data,
-					 unsigned int predicted_us)
+static unsigned int get_typical_interval(struct menu_device *data)
 {
 	int i, divisor;
 	unsigned int min, max, thresh, avg;
@@ -195,11 +196,7 @@ static unsigned int get_typical_interval(struct menu_device *data,
 		}
 	}
 
-	/*
-	 * If the result of the computation is going to be discarded anyway,
-	 * avoid the computation altogether.
-	 */
-	if (min >= predicted_us)
+	if (!max)
 		return UINT_MAX;
 
 	if (divisor == INTERVALS)
@@ -267,7 +264,6 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 {
 	struct menu_device *data = this_cpu_ptr(&menu_devices);
 	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
-	unsigned int predicted_us;
 	u64 predicted_ns;
 	u64 interactivity_req;
 	unsigned int nr_iowaiters;
@@ -279,16 +275,41 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		data->needs_update = 0;
 	}
 
-	/* determine the expected residency time, round up */
-	delta = tick_nohz_get_sleep_length(&delta_tick);
-	if (unlikely(delta < 0)) {
-		delta = 0;
-		delta_tick = 0;
-	}
-	data->next_timer_ns = delta;
-
 	nr_iowaiters = nr_iowait_cpu(dev->cpu);
-	data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
+
+	/* Find the shortest expected idle interval. */
+	predicted_ns = get_typical_interval(data) * NSEC_PER_USEC;
+	if (predicted_ns > RESIDENCY_THRESHOLD_NS) {
+		unsigned int timer_us;
+
+		/* Determine the time till the closest timer. */
+		delta = tick_nohz_get_sleep_length(&delta_tick);
+		if (unlikely(delta < 0)) {
+			delta = 0;
+			delta_tick = 0;
+		}
+
+		data->next_timer_ns = delta;
+		data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
+
+		/* Round up the result for half microseconds. */
+		timer_us = div_u64((RESOLUTION * DECAY * NSEC_PER_USEC) / 2 +
+					data->next_timer_ns *
+						data->correction_factor[data->bucket],
+				   RESOLUTION * DECAY * NSEC_PER_USEC);
+		/* Use the lowest expected idle interval to pick the idle state. */
+		predicted_ns = min((u64)timer_us * NSEC_PER_USEC, predicted_ns);
+	} else {
+		/*
+		 * Because the next timer event is not going to be determined
+		 * in this case, assume that without the tick the closest timer
+		 * will be in distant future and that the closest tick will occur
+		 * after 1/2 of the tick period.
+		 */
+		data->next_timer_ns = KTIME_MAX;
+		delta_tick = TICK_NSEC / 2;
+		data->bucket = which_bucket(KTIME_MAX, nr_iowaiters);
+	}
 
 	if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
 	    ((data->next_timer_ns < drv->states[1].target_residency_ns ||
@@ -303,16 +324,6 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		return 0;
 	}
 
-	/* Round up the result for half microseconds. */
-	predicted_us = div_u64(data->next_timer_ns *
-			       data->correction_factor[data->bucket] +
-			       (RESOLUTION * DECAY * NSEC_PER_USEC) / 2,
-			       RESOLUTION * DECAY * NSEC_PER_USEC);
-	/* Use the lowest expected idle interval to pick the idle state. */
-	predicted_ns = (u64)min(predicted_us,
-				get_typical_interval(data, predicted_us)) *
-				NSEC_PER_USEC;
-
 	if (tick_nohz_tick_stopped()) {
 		/*
 		 * If the tick is already stopped, the cost of possible short

drivers/cpuidle/governors/teo.c

@@ -140,6 +140,8 @@
 #include <linux/sched/topology.h>
 #include <linux/tick.h>
 
+#include "gov.h"
+
 /*
  * The number of bits to shift the CPU's capacity by in order to determine
  * the utilized threshold.
@@ -152,7 +154,6 @@
  */
 #define UTIL_THRESHOLD_SHIFT 6
 
-
 /*
  * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
  * is used for decreasing metrics on a regular basis.
@@ -166,12 +167,6 @@
  */
 #define NR_RECENT	9
 
-/*
- * Idle state target residency threshold used for deciding whether or not to
- * check the time till the closest expected timer event.
- */
-#define RESIDENCY_THRESHOLD_NS	(15 * NSEC_PER_USEC)
-
 /**
  * struct teo_bin - Metrics used by the TEO cpuidle governor.
  * @intercepts: The "intercepts" metric.