linux-kernel - [PATCH 2/2] cpuidle: menu: Avoid computations when result will be discarded

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Message-ID: <3958974.INltPUcQCY@aspire.rjw.lan>
Date:   Mon, 15 Oct 2018 13:53:25 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Doug Smythies <dsmythies@...us.net>
Subject: [PATCH 2/2] cpuidle: menu: Avoid computations when result will be discarded

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

If the minimum interval taken into account in the average computation
loop in get_typical_interval() is less than the expected idle
duration determined so far, the resultant average cannot be greater
than that value as well and the entire return result of the function
is going to be discarded anyway going forward.

In that case, it is a waste of time to carry out the remaining
computations in get_typical_interval(), so avoid that by returning
early if the minimum interval is not below the expected idle duration.

No intentional changes of behavior.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/cpuidle/governors/menu.c |   19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/menu.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/menu.c
+++ linux-pm/drivers/cpuidle/governors/menu.c
@@ -196,10 +196,11 @@ static void menu_update(struct cpuidle_d
  * 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)
+static unsigned int get_typical_interval(struct menu_device *data,
+					 unsigned int predicted_us)
 {
 	int i, divisor;
-	unsigned int max, thresh, avg;
+	unsigned int min, max, thresh, avg;
 	uint64_t sum, variance;
 
 	thresh = UINT_MAX; /* Discard outliers above this value */
@@ -207,6 +208,7 @@ static unsigned int get_typical_interval
 again:
 
 	/* First calculate the average of past intervals */
+	min = UINT_MAX;
 	max = 0;
 	sum = 0;
 	divisor = 0;
@@ -217,8 +219,19 @@ again:
 			divisor++;
 			if (value > max)
 				max = value;
+
+			if (value < min)
+				min = value;
 		}
 	}
+
+	/*
+	 * If the result of the computation is going to be discarded anyway,
+	 * avoid the computation altogether.
+	 */
+	if (min >= predicted_us)
+		return UINT_MAX;
+
 	if (divisor == INTERVALS)
 		avg = sum >> INTERVAL_SHIFT;
 	else
@@ -325,7 +338,7 @@ static int menu_select(struct cpuidle_dr
 	/*
 	 * Use the lowest expected idle interval to pick the idle state.
 	 */
-	predicted_us = min(predicted_us, get_typical_interval(data));
+	predicted_us = min(predicted_us, get_typical_interval(data, predicted_us));
 
 	if (tick_nohz_tick_stopped()) {
 		/*