linux-kernel - [PATCH] watchdog: optimizing the hrtimer interval for power saving

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Date:	Fri, 23 Nov 2012 00:48:26 +0800
From:	Chuansheng Liu <chuansheng.liu@...el.com>
To:	akpm@...ux-foundation.org, dzickus@...hat.com, mingo@...nel.org,
	rjw@...k.pl
Cc:	linux-kernel@...r.kernel.org, chuansheng.liu@...el.com
Subject: [PATCH] watchdog: optimizing the hrtimer interval for power saving


By default, the watchdog threshold is 10, it means every 4s
every CPU will receive one hrtimer interrupt, for low power
device, it will cause 4-5mV power impact when device is deep
sleep.

So here want to optimize it as below:
4s + 4s + 4s + 4s + 4s
== >
12s + 2s + 2s + 2s + 2s
3/5   1/10 1/10 1/10 1/10

In 5 chances, once one chance is hit, then we can start the
hrtimer with a longer period sample(12s). Until the current
chance is not hit, will start the hrtimer with a shorted
period sample(2s).

With this patch, in most case the hrtimer will be 12s instead
of 4s averagely. It can save the device power indeed.

Signed-off-by: liu chuansheng <chuansheng.liu@...el.com>
---
 kernel/watchdog.c |   30 ++++++++++++++++++++++++++++--
 1 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index dd4b80a..6457e62 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -125,7 +125,24 @@ static u64 get_sample_period(void)
 	 * and hard thresholds) to increment before the
 	 * hardlockup detector generates a warning
 	 */
-	return get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 5);
+	return get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 10);
+}
+
+static u64 get_long_sample_period(void)
+{
+	/*
+	 * convert watchdog_thresh from seconds to ns
+	 * We want to give 5 chances to detect softlockup,
+	 * for power saving, once one chance is succeeding,
+	 * we can set long period to avoid power consumption.
+	 * Currently, set the long sample period is:
+	 * 20s * 3/5 = 12s, once this 12s chance is not hit,
+	 * we will divide the left 8s into 4 pieces, give every
+	 * chance every 2s, so it will be likely:
+	 * 12s + 2s + 2s + 2s + 2s,
+	 * Anyway, we just use 12s is enough in normal case.
+	 */
+	return get_softlockup_thresh() * ((u64)NSEC_PER_SEC * 3 / 5);
 }
 
 /* Commands for resetting the watchdog */
@@ -267,6 +284,10 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	unsigned long touch_ts = __this_cpu_read(watchdog_touch_ts);
 	struct pt_regs *regs = get_irq_regs();
 	int duration;
+	bool is_touched;
+
+	is_touched = (__this_cpu_read(hrtimer_interrupts) ==
+		__this_cpu_read(soft_lockup_hrtimer_cnt));
 
 	/* kick the hardlockup detector */
 	watchdog_interrupt_count();
@@ -275,7 +296,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	wake_up_process(__this_cpu_read(softlockup_watchdog));
 
 	/* .. and repeat */
-	hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
+	if (is_touched) {
+		hrtimer_forward_now(hrtimer,
+			ns_to_ktime(get_long_sample_period()));
+	} else {
+		hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
+	}
 
 	if (touch_ts == 0) {
 		if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
-- 
1.7.0.4



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