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

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Date:	Mon, 26 Nov 2012 15:09:17 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Chuansheng Liu <chuansheng.liu@...el.com>
Cc:	akpm@...ux-foundation.org, mingo@...nel.org, rjw@...k.pl,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: optimizing the hrtimer interval for power
 saving

On Fri, Nov 23, 2012 at 12:48:26AM +0800, Chuansheng Liu wrote:
> 
> 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).

Hmm.  Have you tried this patch with cpuspeed (or cpupower) disabled?
The reason I ask is the watchdog threshold is set to 10 which means the
hardlockup watchdog will go off in 10 seconds if it isn't kicked by
hrtimers.

So 12 seconds will miss the window repeatedly.

However, the hardlockup works by calculating the max cpu frequency and
converting it to 10 seconds.  Thanks to cpuspeed, most machines don't run
at max frequency.  Therefore a 10 second window is usually 60 seconds or
more.   So your initial testing might have missed the fact that 12 seconds
is greater than the 10 second hardlockup period.

Cheers,
Don

> 
> 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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