Re: [PATCH] tick: make sleep length calculation more accurate

[Daniel Lezcano]

On 10/01/2013 11:31 PM, Stephen Boyd wrote:
> On 09/27/13 03:52, Daniel Lezcano wrote:
> > The sleep_length is computed in the tick_nohz_stop_sched_tick function but it
> > is used later in the code with in between the local irq enabled.
> >
> > cpu_idle_loop
> >    tick_nohz_idle_enter         [ exits with local irq enabled ]
> >     __tick_nohz_idle_enter
> >       tick_nohz_stop_sched_tick
> >    ...
> >
> >    arch_cpu_idle
> >       menu_select               [ uses here 'sleep_length' ]
> >    ...
> >
> > Between the computation of the sleep length and its usage, some interrupts
> > can occur, making the sleep length shorter than actually it is.
> >
> > This patch fixes that by moving the sleep_length computation in the
> > tick_nohz_get_sleep_length function and store the next_event for the device
> > instead of the sleep_length.
> >
> > Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > ---
> >   include/linux/tick.h     |    2 +-
> >   kernel/time/tick-sched.c |    5 +++--
> >   2 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index 5128d33..4932004 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -67,7 +67,7 @@ struct tick_sched {
> >   	ktime_t				idle_exittime;
> >   	ktime_t				idle_sleeptime;
> >   	ktime_t				iowait_sleeptime;
> > -	ktime_t				sleep_length;
> > +	ktime_t				next_event;
> >   	unsigned long			last_jiffies;
> >   	unsigned long			next_jiffies;
> >   	ktime_t				idle_expires;
>
> Documentation update?
>
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 3612fc7..2007a7f 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -673,7 +673,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >   out:
> >   	ts->next_jiffies = next_jiffies;
> >   	ts->last_jiffies = last_jiffies;
> > -	ts->sleep_length = ktime_sub(dev->next_event, now);
> > +	ts->next_event = dev->next_event;
> >
> >   	return ret;
> >   }
> > @@ -837,8 +837,9 @@ void tick_nohz_irq_exit(void)
> >   ktime_t tick_nohz_get_sleep_length(void)
> >   {
> >   	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> > +	ktime_t now = ktime_get();
> >
> > -	return ts->sleep_length;
> > +	return ktime_sub(ts->next_event, now);
> >   }
> >
> >   static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
>
> What happens if the idling CPU's next_event is updated via that
> interrupt? Say if the interrupt handler schedules a timer to fire before
> the next timer on the CPU? It looks like we won't notice that.

Yes, or after.

It sounds like this issue also occurs with the current code, no ?

> Perhaps it's better to do this instead?
>
>   ktime_t tick_nohz_get_sleep_length(void)
>   {
>   	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> +	ktime_t now = ktime_get();
> +	struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
>
> -	return ts->sleep_length;
> +	return ktime_sub(dev->next_event, now);
>   }

Yes, I agree.

Thanks for the review.

  -- Daniel


--
 <http://www.linaro.org/> Linaro.org â Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

--
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/