linux-kernel - Re: [PATCH v7 06/26] timer: Export next wakeup time of a CPU

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Date:   Fri, 13 Apr 2018 10:47:39 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux PM <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 <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v7 06/26] timer: Export next wakeup time of a CPU

On Thu, Apr 12, 2018 at 1:14 PM, Ulf Hansson <ulf.hansson@...aro.org> 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     | 10 ++++++++++
>  kernel/time/tick-sched.c | 11 +++++++++++
>  2 files changed, 21 insertions(+)
>
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 389aa25..d341811 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 +152,15 @@ 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)
> +{
> +       ktime_t t;
> +
> +       /* Next wake up is the tick period, assume it starts now */
> +       return ktime_add(tick_nohz_get_sleep_length(&t), ktime_get());
> +}

Well, given that tick_nohz_get_sleep_length() is just the above in
this case, wouldn't it be simpler to return ktime_add(ktime_get(),
TICK_NSEC) from here?

> +
>  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 646645e..08db7f3 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1098,6 +1098,17 @@ 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
> + */
> +ktime_t tick_nohz_get_next_wakeup(int cpu)
> +{
> +       struct clock_event_device *dev =
> +                       per_cpu(tick_cpu_device.evtdev, cpu);

I would avoid breaking the line, honestly.

If you really really want to avoid going above the 80 chars line
length limit, why don't you do

struct clock_event_device *dev;

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
> --