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

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Date:	Tue, 01 Oct 2013 14:31:00 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] tick: make sleep length calculation more accurate

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@...aro.org>
> ---
>  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.

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);
 }


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