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Date:	Mon, 10 Mar 2014 12:54:59 +0200
From:	Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
To:	Len Brown <lenb@...nel.org>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] Cpuidle: Deal with timer expiring in the past

Hi,

On 6 March 2014 09:41, Len Brown <lenb@...nel.org> wrote:
>
> On Mon, Feb 24, 2014 at 1:29 AM, Tuukka Tikkanen
> <tuukka.tikkanen@...aro.org> wrote:
> > Sometimes (fairly often) when the cpuidle menu governor is making a decision
> > about idle state to enter the next timer for the cpu appears to expire in
> > the past. The menu governor expects the expiry to always be in the future
> > and in fact stores the time delta in an unsigned variable. However, when
> > the expiry is in the past, the value returned by tick_nohz_get_sleep_length
> > can be negative. This patch prevents using negative values, instead making
> > the governor return immediately similar to having latency requirement set
> > to 0.
> >
> > Note: As with latency == 0, the return value is 0 with no check to see if
> > the state 0 has been disabled or not.
> >
> > Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
> > ---
> >  drivers/cpuidle/governors/menu.c |   10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> > index 71b5232..c414468 100644
> > --- a/drivers/cpuidle/governors/menu.c
> > +++ b/drivers/cpuidle/governors/menu.c
> > @@ -302,8 +302,16 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> >         if (unlikely(latency_req == 0))
> >                 return 0;
> >
> > -       /* determine the expected residency time, round up */
> > +       /*
> > +        * Determine the expected residency time. If the time is negative,
> > +        * a timer interrupt has probably just expired after disabling
> > +        * interrupts. Return as quickly as possible in the most shallow
> > +        * state possible. tv_nsec is always positive, so only check the
> > +        * seconds.
> > +        */
> >         t = ktime_to_timespec(tick_nohz_get_sleep_length());
> > +       if (t.tv_sec < 0)
> > +               return 0;
> >         data->next_timer_us =
> >                 t.tv_sec * USEC_PER_SEC + t.tv_nsec / NSEC_PER_USEC;
> >
>
> Are there special conditions that are necessary to provoke a negative
> return value?
> I've traced this code on several systems, and never seen a negative
> return value.
>

By changing the if statement to
if (WARN_ONCE(t.tv_sec < 0, "Next timer in the past. tv_sec: %ld,
tv_usec: %lu", t.tv_sec, t.tv_usec)) return 0;

I eventually got
[ 3092.355144] WARNING: CPU: 4 PID: 0 at
drivers/cpuidle/governors/menu.c:315 menu_select+0x410/0x420()
[ 3092.355145] Next timer in the past. tv_sec: -1, tv_usec: 999999645
...
[ 3092.355169] CPU: 4 PID: 0 Comm: swapper/4 Not tainted 3.14.0-rc3+ #3

so the problem is still lurking there. However, as you can see from
the timestamp, either something has changed recently and made it much
much less frequent or I did not use the use case / hardware that
triggers it more often. It has been quite a long time since I
discovered and fixed this in my private trees, so I have forgotten the
best way to reproduce this, sorry. In any case, existence is still
confirmed.

This was on a i7-3770K with 64 bit kernel, running make clean && make
-j8 && make clean && make for kernel source tree. If I happen to
remember the use case + hardware combination that triggers this
rapidly, I'll let you know.

That said, I agree dropping this patch and fixing the called function
is a better idea.

Tuukka

>
> However...
> I do see values up to 300.2 seconds, and those large values seem to decay
> at the rate of real-time so that after 5 minutes they are small, and then
> jump back up to 300 seconds.
>
> Some folks at Oracle debugged it down to use of NEXT_TIMER_MAX_DELTA
> when there is _no_ timer currently pending on that CPU.  It seems this is easier
> to observe, the more CPUs a system has -- though I've been able to reproduce
> it on a system as small as a single-package 8-cpu systems.
>
> One proposed way to address this is to cap large values at 1 second.
> However, that will not recognize that for the period when the large
> value decays to under 1 second, all of those are fiction.
>
> Also, if we could identify the case where there is no future timer,
> it seems that re-using dev->last_residency would probably be
> a more useful guess than pretending we'll have a timer expire in 1 second.
>
> thanks,
> Len Brown, Intel Open Source Technology Cente
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