RE: [PATCH] watchdog: optimizing the hrtimer interval for powersaving

[Liu, Chuansheng]

> -----Original Message-----
> From: Don Zickus [mailto:dzickus@xxxxxxxxxx]
> Sent: Tuesday, November 27, 2012 4:09 AM
> To: Liu, Chuansheng
> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; mingo@xxxxxxxxxx; rjw@xxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> 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.
Thanks your pointing out. 
So I will submit V2 patch with the below checking about starting 2s or 12s hrtimer:
void watchdog_timer_fn()
{
  If(watchdog_nmi_touch is false)
     start 2s timer;
  else
     start 12s timer;
}
It can cover the case of 12s > 10s although in most cases it will not happen,
as you said, the 10s window is usually 60s.

> 
> 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.
I have tried the case of set the CPU frequency to MAX, the nmi interrupt interval
is still about 20s.
> 
> 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@xxxxxxxxx>
> > ---
> >  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
> >
> >
> >
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/