Re: [PATCH 08/14] timer: Export next wake up of a CPU

[Kevin Hilman <khilman@baylibre.com>]

Lina Iyer <lina.iyer@linaro.org> writes:

> Knowing the sleep length of the CPU is useful for the power state
> determination on idle. However, when the common sleep time between
> multiple CPUs is needed, the sleep length of a CPU is not useful.

You need more about "why" it's not useful.  e.g. CPU sleep length is
relative to when that CPU entered idle, whereas when trying to determine
cluster sleep time, absolute times are needed, like dev->next_event.

> By reading the next wake up event of a CPU, governors can determine the
> first CPU to wake up (due to timer) amongst a cluster of CPUs and the
> sleep time available between the last CPU to idle and the first CPU to
> resume. This information is useful to determine if the caches and other
> common hardware blocks can also be put in idle during this common period
> of inactivity.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Lina Iyer <lina.iyer@linaro.org>

Otherwise, looks OK to me.

Reviewed-by: Kevin Hilman <khilman@baylibre.com>

> ---
>  include/linux/tick.h     | 10 ++++++++++
>  kernel/time/tick-sched.c | 13 +++++++++++++
>  2 files changed, 23 insertions(+)
>
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 62be0786..9535141 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -117,6 +117,7 @@ extern void tick_nohz_idle_enter(void);
>  extern void tick_nohz_idle_exit(void);
>  extern void tick_nohz_irq_exit(void);
>  extern ktime_t tick_nohz_get_sleep_length(void);
> +extern ktime_t tick_nohz_get_next_wakeup(int cpu);
>  extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
>  extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
>  #else /* !CONFIG_NO_HZ_COMMON */
> @@ -131,6 +132,15 @@ static inline ktime_t tick_nohz_get_sleep_length(void)
>  
>  	return len;
>  }
> +
> +static inline ktime_t tick_nohz_get_next_wakeup(int cpu)
> +{
> +	ktime_t len = { .tv64 = NSEC_PER_SEC/HZ };
> +
> +	/* Next wake up is the tick period, assume it starts now */
> +	return ktime_add(len, ktime_get());
> +}
> +
>  static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
>  static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
>  #endif /* !CONFIG_NO_HZ_COMMON */
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 536ada8..addf3235 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -979,6 +979,19 @@ ktime_t tick_nohz_get_sleep_length(void)
>  	return ts->sleep_length;
>  }
>  
> +/**
> + * tick_nohz_get_next_wakeup - return the next wake up of the CPU
> + *
> + * Called with interrupts disabled on the cpu
> + */
> +ktime_t tick_nohz_get_next_wakeup(int cpu)
> +{
> +	struct clock_event_device *dev =
> +			per_cpu(tick_cpu_device.evtdev, cpu);
> +
> +	return dev->next_event;
> +}
> +
>  static void tick_nohz_account_idle_ticks(struct tick_sched *ts)
>  {
>  #ifndef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE