cpuidle: Add decaying history logic to menu idle predictor

Add decaying history of predicted idle time, instead of using the last early
wakeup. This logic helps menu governor do better job of predicting idle time.

With this change, we also measured noticable (~8%) power savings on
a DP server system with CPUs supporting deep C states, when system
was lightly loaded. There was no change to power or perf on other load
conditions.

Signed-off-by: Venkatesh Pallipadi <[e-mail]>
Signed-off-by: Len Brown <[e-mail]>

drivers/cpuidle/governors/menu.c

···
 #include <linux/tick.h>
 
 #define BREAK_FUZZ	4	/* 4 us */
+#define PRED_HISTORY_PCT	50
 
 struct menu_device {
 	int		last_state_idx;
 
 	unsigned int	expected_us;
 	unsigned int	predicted_us;
+	unsigned int    current_predicted_us;
 	unsigned int	last_measured_us;
 	unsigned int	elapsed_us;
 };
···
 	/* determine the expected residency time */
 	data->expected_us =
 		(u32) ktime_to_ns(tick_nohz_get_sleep_length()) / 1000;
+
+	/* Recalculate predicted_us based on prediction_history_pct */
+	data->predicted_us *= PRED_HISTORY_PCT;
+	data->predicted_us += (100 - PRED_HISTORY_PCT) *
+				data->current_predicted_us;
+	data->predicted_us /= 100;
 
 	/* find the deepest idle state that satisfies our constraints */
 	for (i = CPUIDLE_DRIVER_STATE_START + 1; i < dev->state_count; i++) {
···
 		measured_us = -1;
 
 	/* Predict time until next break event */
-	data->predicted_us = max(measured_us, data->last_measured_us);
+	data->current_predicted_us = max(measured_us, data->last_measured_us);
 
 	if (last_idle_us + BREAK_FUZZ <
 	    data->expected_us - target->exit_latency) {