linux-kernel - [PATCH 2/5] x86,idle: Quickly notice prediction failure in general case

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Date:	Tue, 16 Oct 2012 21:04:37 -0400
From:	Youquan Song <youquan.song@...el.com>
To:	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	arjan@...ux.intel.com, lenb@...nel.org
Cc:	Rik van Riel <riel@...hat.com>,
	Youquan Song <youquan.song@...ux.intel.com>,
	Youquan Song <youquan.song@...el.com>
Subject: [PATCH 2/5] x86,idle: Quickly notice prediction failure in general case

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 to general case that prediction logic get a small predicted
residency, so it choose a shallow C-state though the expected residency is large
. Once the prediction will be fail, the CPU will keep staying at shallow C-state
for a long time. Acutally, the CPU has change enter into deep C-state.
So when the expected residency is long enough but governor choose a shallow
C-state, an timer will be added in order to monitor if the prediction failure. 

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@...el.com>
Signed-off-by: Rik van Riel <riel@...hat.com>
---
 drivers/cpuidle/governors/menu.c |   22 ++++++++++++++++++++++
 1 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index beeab6a..b34bf11 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -114,6 +114,13 @@ static DEFINE_PER_CPU(int, hrtimer_started);
  *
  */
 
+/*
+ * The C-state residency is so long that is is worthwhile to exit
+ * from the shallow C-state and re-enter into a deeper C-state.
+ */
+static unsigned int perfect_cstate_ms __read_mostly = 30;
+module_param(perfect_cstate_ms, uint, 0000);
+
 struct menu_device {
 	int		last_state_idx;
 	int             needs_update;
@@ -351,6 +358,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 	/* not deepest C-state chosen for low predicted residency */
 	if (low_predicted) {
 		unsigned int timer_us = 0;
+		unsigned int perfect_us = 0;
 
 		/*
 		 * Set a timer to detect whether this sleep is much
@@ -361,12 +369,26 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 		 */
 		timer_us = 2 * (data->predicted_us + MAX_DEVIATION);
 
+		perfect_us = perfect_cstate_ms * 1000;
+
 		if (repeat && (4 * timer_us < data->expected_us)) {
 			hrtimer_start(hrtmr, ns_to_ktime(1000 * timer_us),
 				HRTIMER_MODE_REL_PINNED);
 			/* menu hrtimer is started */
 			per_cpu(hrtimer_started, cpu) = 1;
+		} else if (perfect_us < data->expected_us) {
+			/*
+			 * The next timer is long. This could be because
+			 * we did not make a useful prediction.
+			 * In that case, it makes sense to re-enter
+			 * into a deeper C-state after some time.
+			 */
+			hrtimer_start(hrtmr, ns_to_ktime(1000 * timer_us),
+				HRTIMER_MODE_REL_PINNED);
+			/* menu hrtimer is started */
+			per_cpu(hrtimer_started, cpu) = 1;
 		}
+
 	}
 
 	return data->last_state_idx;
-- 
1.7.7.4

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