[PATCH V2 2/3] x86,idle: Quickly notice prediction failure in general case

[Youquan Song <youquan.song@intel.com>]

The prediction for future is difficult and when the cpuidle governor prediction 
fails and govenor possibly choose the shallower C-state than it should. How to 
quickly notice and find the failure becomes important for power saving.    

The patch extends the patch to enhance the prediction for repeat mode by add a 
timer when menu governor choose a shallow C-state. 
The timer is set to time out in 50 milli-seconds by default. It is special twist
 that there are no power saving gains even sleep longer than it.
  
When C-state is waken up prior to the adding timer, the timer will be cancelled 
initiatively. When the timer is triggered and menu governor will quickly notice
prediction failure and re-evaluates deeper C-states possibility. 

Signed-off-by: Youquan Song <youquan.song@intel.com>
---
 drivers/cpuidle/governors/menu.c |   48 ++++++++++++++++++++++++++------------
 1 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 8c23fbd..9f92dd4 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -113,6 +113,13 @@ static DEFINE_PER_CPU(int, hrtimer_started);
  * represented in the system load average.
  *
  */
+
+/*
+ * Default set to 50 milliseconds based on special twist mentioned above that
+ * there are no power gains sleep longer than it.
+ */
+static unsigned int perfect_cstate_ms __read_mostly = 50;
+module_param(perfect_cstate_ms, uint, 0000);
 
 struct menu_device {
 	int		last_state_idx;
@@ -343,26 +350,37 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 			data->exit_us = s->exit_latency;
 		}
 	}
-
+
+	/* not deepest C-state chosen */
 	if (data->last_state_idx < drv->state_count - 1) { 
+		unsigned int repeat_us = 0;
+		unsigned int perfect_us = 0;
+
+		/*
+		 * Set enough timer to recognize the repeat mode broken.
+		 * If the timer is time out, the repeat mode prediction
+		 * fails,then re-evaluate deeper C-states possibility.
+		 * If the timer is not triggered, the timer will be
+		 * cancelled when CPU waken up.
+		 */
+		repeat_us =
+			(repeat ? (2 * data->predicted_us + MAX_DEVIATION) : 0);
+		perfect_us = perfect_cstate_ms * 1000;
 
 		/* Repeat mode detected */
-		if (repeat) {
-			unsigned int repeat_us = 0;
-			/* 
-			 * Set enough timer to recognize the repeat mode broken.
-			 * If the timer is time out, the repeat mode prediction
-			 * fails,then re-evaluate deeper C-states possibility. 
-			 * If the timer is not triggered, the timer will be
-			 * cancelled when CPU waken up.
-			 */
-			repeat_us = 2 * data->predicted_us + MAX_DEVIATION;
-			hrtimer_start(hrtmr, ns_to_ktime(1000 * repeat_us),
-	      			HRTIMER_MODE_REL_PINNED);
+		if (repeat && (repeat_us  < perfect_us)) {
+			hrtimer_start(hrtmr, ns_to_ktime(1000 * repeat_us),
+				HRTIMER_MODE_REL_PINNED);
+			/* menu hrtimer is started */
+			per_cpu(hrtimer_started, cpu) = 1;
+		} else if (perfect_us < data->expected_us) {
+			/* expected time is larger than adding timer time */
+			hrtimer_start(hrtmr, ns_to_ktime(1000 * perfect_us),
+					HRTIMER_MODE_REL_PINNED);
 			/* menu hrtimer is started */
 			per_cpu(hrtimer_started, cpu) = 1;
-		}
-	}
+		}
+	}
 
 	return data->last_state_idx;
 }
-- 
1.6.4.2