linux-kernel - Re: [PATCH v8 06/26] timer: Export next wakeup time of a CPU

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Date:   Thu, 19 Jul 2018 12:15:57 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>, linux-pm@...r.kernel.org,
        Kevin Hilman <khilman@...nel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Lina Iyer <lina.iyer@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Juri Lelli <juri.lelli@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v8 06/26] timer: Export next wakeup time of a CPU

On Wednesday, June 20, 2018 7:22:06 PM CEST Ulf Hansson wrote:
> From: Lina Iyer <lina.iyer@...aro.org>
> 
> Knowing the sleep duration of CPUs, is known to be needed while selecting
> the most energy efficient idle state for a CPU or a group of CPUs.
> 
> However, to be able to compute the sleep duration, we need to know at what
> time the next expected wakeup is for the CPU. Therefore, let's export this
> information via a new function, tick_nohz_get_next_wakeup(). Following
> changes make use of it.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Lina Iyer <ilina@...eaurora.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Lina Iyer <lina.iyer@...aro.org>
> Co-developed-by: Ulf Hansson <ulf.hansson@...aro.org>
> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> ---
>  include/linux/tick.h     |  8 ++++++++
>  kernel/time/tick-sched.c | 10 ++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 55388ab45fd4..e48f6b26b425 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -125,6 +125,7 @@ extern bool tick_nohz_idle_got_tick(void);
>  extern ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next);
>  extern unsigned long tick_nohz_get_idle_calls(void);
>  extern unsigned long tick_nohz_get_idle_calls_cpu(int cpu);
> +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);
>  
> @@ -151,6 +152,13 @@ static inline ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
>  	*delta_next = TICK_NSEC;
>  	return *delta_next;
>  }
> +
> +static inline ktime_t tick_nohz_get_next_wakeup(int cpu)
> +{
> +	/* Next wake up is the tick period, assume it starts now */
> +	return ktime_add(ktime_get(), TICK_NSEC);
> +}
> +
>  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; }
>  
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index da9455a6b42b..f380bb4f0744 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1089,6 +1089,16 @@ unsigned long tick_nohz_get_idle_calls(void)
>  	return ts->idle_calls;
>  }
>  
> +/**
> + * tick_nohz_get_next_wakeup - return the next wake up of the CPU
> + */

I'd add to the comment that this is to be invoked for idle CPUs only.

> +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
>