Linux 3.16.66

[linux/fpc-iii.git] / kernel / time / tick-sched.c

/*
 *  linux/kernel/time/tick-sched.c
 *
 *  Copyright(C) 2005-2006, Thomas Gleixner <tglx@linutronix.de>
 *  Copyright(C) 2005-2007, Red Hat, Inc., Ingo Molnar
 *  Copyright(C) 2006-2007  Timesys Corp., Thomas Gleixner
 *
 *  No idle tick implementation for low and high resolution timers
 *
 *  Started by: Thomas Gleixner and Ingo Molnar
 *
 *  Distribute under GPLv2.
 */
#include <linux/cpu.h>
#include <linux/err.h>
#include <linux/hrtimer.h>
#include <linux/interrupt.h>
#include <linux/kernel_stat.h>
#include <linux/percpu.h>
#include <linux/profile.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/irq_work.h>
#include <linux/posix-timers.h>
#include <linux/perf_event.h>
#include <linux/context_tracking.h>

#include <asm/irq_regs.h>

#include "tick-internal.h"

#include <trace/events/timer.h>

/*
 * Per cpu nohz control structure
 */
DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);

/*
 * The time, when the last jiffy update happened. Protected by jiffies_lock.
 */
static ktime_t last_jiffies_update;

struct tick_sched *tick_get_tick_sched(int cpu)
{
        return &per_cpu(tick_cpu_sched, cpu);
}

/*
 * Must be called with interrupts disabled !
 */
static void tick_do_update_jiffies64(ktime_t now)
{
        unsigned long ticks = 0;
        ktime_t delta;

        /*
         * Do a quick check without holding jiffies_lock:
         */
        delta = ktime_sub(now, last_jiffies_update);
        if (delta.tv64 < tick_period.tv64)
                return;

        /* Reevalute with jiffies_lock held */
        write_seqlock(&jiffies_lock);

        delta = ktime_sub(now, last_jiffies_update);
        if (delta.tv64 >= tick_period.tv64) {

                delta = ktime_sub(delta, tick_period);
                last_jiffies_update = ktime_add(last_jiffies_update,
                                                tick_period);

                /* Slow path for long timeouts */
                if (unlikely(delta.tv64 >= tick_period.tv64)) {
                        s64 incr = ktime_to_ns(tick_period);

                        ticks = ktime_divns(delta, incr);

                        last_jiffies_update = ktime_add_ns(last_jiffies_update,
                                                           incr * ticks);
                }
                do_timer(++ticks);

                /* Keep the tick_next_period variable up to date */
                tick_next_period = ktime_add(last_jiffies_update, tick_period);
        } else {
                write_sequnlock(&jiffies_lock);
                return;
        }
        write_sequnlock(&jiffies_lock);
        update_wall_time();
}

/*
 * Initialize and return retrieve the jiffies update.
 */
static ktime_t tick_init_jiffy_update(void)
{
        ktime_t period;

        write_seqlock(&jiffies_lock);
        /* Did we start the jiffies update yet ? */
        if (last_jiffies_update.tv64 == 0)
                last_jiffies_update = tick_next_period;
        period = last_jiffies_update;
        write_sequnlock(&jiffies_lock);
        return period;
}


static void tick_sched_do_timer(ktime_t now)
{
        int cpu = smp_processor_id();

#ifdef CONFIG_NO_HZ_COMMON
        /*
         * Check if the do_timer duty was dropped. We don't care about
         * concurrency: This happens only when the cpu in charge went
         * into a long sleep. If two cpus happen to assign themself to
         * this duty, then the jiffies update is still serialized by
         * jiffies_lock.
         */
        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)
            && !tick_nohz_full_cpu(cpu))
                tick_do_timer_cpu = cpu;
#endif

        /* Check, if the jiffies need an update */
        if (tick_do_timer_cpu == cpu)
                tick_do_update_jiffies64(now);
}

static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
{
#ifdef CONFIG_NO_HZ_COMMON
        /*
         * When we are idle and the tick is stopped, we have to touch
         * the watchdog as we might not schedule for a really long
         * time. This happens on complete idle SMP systems while
         * waiting on the login prompt. We also increment the "start of
         * idle" jiffy stamp so the idle accounting adjustment we do
         * when we go busy again does not account too much ticks.
         */
        if (ts->tick_stopped) {
                touch_softlockup_watchdog();
                if (is_idle_task(current))
                        ts->idle_jiffies++;
        }
#endif
        update_process_times(user_mode(regs));
        profile_tick(CPU_PROFILING);
}

#ifdef CONFIG_NO_HZ_FULL
cpumask_var_t tick_nohz_full_mask;
bool tick_nohz_full_running;

static bool can_stop_full_tick(void)
{
        WARN_ON_ONCE(!irqs_disabled());

        if (!sched_can_stop_tick()) {
                trace_tick_stop(0, "more than 1 task in runqueue\n");
                return false;
        }

        if (!posix_cpu_timers_can_stop_tick(current)) {
                trace_tick_stop(0, "posix timers running\n");
                return false;
        }

        if (!perf_event_can_stop_tick()) {
                trace_tick_stop(0, "perf events running\n");
                return false;
        }

        /* sched_clock_tick() needs us? */
#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
        /*
         * TODO: kick full dynticks CPUs when
         * sched_clock_stable is set.
         */
        if (!sched_clock_stable()) {
                trace_tick_stop(0, "unstable sched clock\n");
                /*
                 * Don't allow the user to think they can get
                 * full NO_HZ with this machine.
                 */
                WARN_ONCE(tick_nohz_full_running,
                          "NO_HZ FULL will not work with unstable sched clock");
                return false;
        }
#endif

        return true;
}

static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now);

/*
 * Re-evaluate the need for the tick on the current CPU
 * and restart it if necessary.
 */
void __tick_nohz_full_check(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        if (tick_nohz_full_cpu(smp_processor_id())) {
                if (ts->tick_stopped && !is_idle_task(current)) {
                        if (!can_stop_full_tick())
                                tick_nohz_restart_sched_tick(ts, ktime_get());
                }
        }
}

static void nohz_full_kick_work_func(struct irq_work *work)
{
        __tick_nohz_full_check();
}

static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
        .func = nohz_full_kick_work_func,
};

/*
 * Kick the current CPU if it's full dynticks in order to force it to
 * re-evaluate its dependency on the tick and restart it if necessary.
 */
void tick_nohz_full_kick(void)
{
        if (tick_nohz_full_cpu(smp_processor_id()))
                irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
}

static void nohz_full_kick_ipi(void *info)
{
        __tick_nohz_full_check();
}

/*
 * Kick all full dynticks CPUs in order to force these to re-evaluate
 * their dependency on the tick and restart it if necessary.
 */
void tick_nohz_full_kick_all(void)
{
        if (!tick_nohz_full_running)
                return;

        preempt_disable();
        smp_call_function_many(tick_nohz_full_mask,
                               nohz_full_kick_ipi, NULL, false);
        tick_nohz_full_kick();
        preempt_enable();
}

/*
 * Re-evaluate the need for the tick as we switch the current task.
 * It might need the tick due to per task/process properties:
 * perf events, posix cpu timers, ...
 */
void __tick_nohz_task_switch(struct task_struct *tsk)
{
        unsigned long flags;

        local_irq_save(flags);

        if (!tick_nohz_full_cpu(smp_processor_id()))
                goto out;

        if (tick_nohz_tick_stopped() && !can_stop_full_tick())
                tick_nohz_full_kick();

out:
        local_irq_restore(flags);
}

/* Parse the boot-time nohz CPU list from the kernel parameters. */
static int __init tick_nohz_full_setup(char *str)
{
        int cpu;

        alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
        if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
                pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
                return 1;
        }

        cpu = smp_processor_id();
        if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
                pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
                cpumask_clear_cpu(cpu, tick_nohz_full_mask);
        }
        tick_nohz_full_running = true;

        return 1;
}
__setup("nohz_full=", tick_nohz_full_setup);

static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
                                                 unsigned long action,
                                                 void *hcpu)
{
        unsigned int cpu = (unsigned long)hcpu;

        switch (action & ~CPU_TASKS_FROZEN) {
        case CPU_DOWN_PREPARE:
                /*
                 * If we handle the timekeeping duty for full dynticks CPUs,
                 * we can't safely shutdown that CPU.
                 */
                if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
                        return NOTIFY_BAD;
                break;
        }
        return NOTIFY_OK;
}

/*
 * Worst case string length in chunks of CPU range seems 2 steps
 * separations: 0,2,4,6,...
 * This is NR_CPUS + sizeof('\0')
 */
static char __initdata nohz_full_buf[NR_CPUS + 1];

static int tick_nohz_init_all(void)
{
        int err = -1;

#ifdef CONFIG_NO_HZ_FULL_ALL
        if (!alloc_cpumask_var(&tick_nohz_full_mask, GFP_KERNEL)) {
                pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
                return err;
        }
        err = 0;
        cpumask_setall(tick_nohz_full_mask);
        cpumask_clear_cpu(smp_processor_id(), tick_nohz_full_mask);
        tick_nohz_full_running = true;
#endif
        return err;
}

void __init tick_nohz_init(void)
{
        int cpu;

        if (!tick_nohz_full_running) {
                if (tick_nohz_init_all() < 0)
                        return;
        }

        for_each_cpu(cpu, tick_nohz_full_mask)
                context_tracking_cpu_set(cpu);

        cpu_notifier(tick_nohz_cpu_down_callback, 0);
        cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), tick_nohz_full_mask);
        pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);
}
#endif

/*
 * NOHZ - aka dynamic tick functionality
 */
#ifdef CONFIG_NO_HZ_COMMON
/*
 * NO HZ enabled ?
 */
static int tick_nohz_enabled __read_mostly  = 1;
int tick_nohz_active  __read_mostly;
/*
 * Enable / Disable tickless mode
 */
static int __init setup_tick_nohz(char *str)
{
        if (!strcmp(str, "off"))
                tick_nohz_enabled = 0;
        else if (!strcmp(str, "on"))
                tick_nohz_enabled = 1;
        else
                return 0;
        return 1;
}

__setup("nohz=", setup_tick_nohz);

/**
 * tick_nohz_update_jiffies - update jiffies when idle was interrupted
 *
 * Called from interrupt entry when the CPU was idle
 *
 * In case the sched_tick was stopped on this CPU, we have to check if jiffies
 * must be updated. Otherwise an interrupt handler could use a stale jiffy
 * value. We do this unconditionally on any cpu, as we don't know whether the
 * cpu, which has the update task assigned is in a long sleep.
 */
static void tick_nohz_update_jiffies(ktime_t now)
{
        unsigned long flags;

        __this_cpu_write(tick_cpu_sched.idle_waketime, now);

        local_irq_save(flags);
        tick_do_update_jiffies64(now);
        local_irq_restore(flags);

        touch_softlockup_watchdog();
}

/*
 * Updates the per cpu time idle statistics counters
 */
static void
update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_update_time)
{
        ktime_t delta;

        if (ts->idle_active) {
                delta = ktime_sub(now, ts->idle_entrytime);
                if (nr_iowait_cpu(cpu) > 0)
                        ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
                else
                        ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
                ts->idle_entrytime = now;
        }

        if (last_update_time)
                *last_update_time = ktime_to_us(now);

}

static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
{
        update_ts_time_stats(smp_processor_id(), ts, now, NULL);
        ts->idle_active = 0;

        sched_clock_idle_wakeup_event(0);
}

static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
{
        ktime_t now = ktime_get();

        ts->idle_entrytime = now;
        ts->idle_active = 1;
        sched_clock_idle_sleep_event();
        return now;
}

/**
 * get_cpu_idle_time_us - get the total idle time of a cpu
 * @cpu: CPU number to query
 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
 *
 * Return the cummulative idle time (since boot) for a given
 * CPU, in microseconds.
 *
 * This time is measured via accounting rather than sampling,
 * and is as accurate as ktime_get() is.
 *
 * This function returns -1 if NOHZ is not enabled.
 */
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        ktime_t now, idle;

        if (!tick_nohz_active)
                return -1;

        now = ktime_get();
        if (last_update_time) {
                update_ts_time_stats(cpu, ts, now, last_update_time);
                idle = ts->idle_sleeptime;
        } else {
                if (ts->idle_active && !nr_iowait_cpu(cpu)) {
                        ktime_t delta = ktime_sub(now, ts->idle_entrytime);

                        idle = ktime_add(ts->idle_sleeptime, delta);
                } else {
                        idle = ts->idle_sleeptime;
                }
        }

        return ktime_to_us(idle);

}
EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);

/**
 * get_cpu_iowait_time_us - get the total iowait time of a cpu
 * @cpu: CPU number to query
 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
 *
 * Return the cummulative iowait time (since boot) for a given
 * CPU, in microseconds.
 *
 * This time is measured via accounting rather than sampling,
 * and is as accurate as ktime_get() is.
 *
 * This function returns -1 if NOHZ is not enabled.
 */
u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        ktime_t now, iowait;

        if (!tick_nohz_active)
                return -1;

        now = ktime_get();
        if (last_update_time) {
                update_ts_time_stats(cpu, ts, now, last_update_time);
                iowait = ts->iowait_sleeptime;
        } else {
                if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
                        ktime_t delta = ktime_sub(now, ts->idle_entrytime);

                        iowait = ktime_add(ts->iowait_sleeptime, delta);
                } else {
                        iowait = ts->iowait_sleeptime;
                }
        }

        return ktime_to_us(iowait);
}
EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);

static inline bool local_timer_softirq_pending(void)
{
        return local_softirq_pending() & BIT(TIMER_SOFTIRQ);
}

static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
                                         ktime_t now, int cpu)
{
        unsigned long seq, last_jiffies, next_jiffies, delta_jiffies;
        ktime_t last_update, expires, ret = { .tv64 = 0 };
        unsigned long rcu_delta_jiffies;
        struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
        u64 time_delta;

        time_delta = timekeeping_max_deferment();

        /* Read jiffies and the time when jiffies were updated last */
        do {
                seq = read_seqbegin(&jiffies_lock);
                last_update = last_jiffies_update;
                last_jiffies = jiffies;
        } while (read_seqretry(&jiffies_lock, seq));

        /*
         * Keep the periodic tick, when RCU, architecture or irq_work
         * requests it.
         * Aside of that check whether the local timer softirq is
         * pending. If so its a bad idea to call get_next_timer_interrupt()
         * because there is an already expired timer, so it will request
         * immeditate expiry, which rearms the hardware timer with a
         * minimal delta which brings us back to this place
         * immediately. Lather, rinse and repeat...
         */
        if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) || arch_needs_cpu(cpu) ||
            irq_work_needs_cpu() || local_timer_softirq_pending()) {
                next_jiffies = last_jiffies + 1;
                delta_jiffies = 1;
        } else {
                /* Get the next timer wheel timer */
                next_jiffies = get_next_timer_interrupt(last_jiffies);
                delta_jiffies = next_jiffies - last_jiffies;
                if (rcu_delta_jiffies < delta_jiffies) {
                        next_jiffies = last_jiffies + rcu_delta_jiffies;
                        delta_jiffies = rcu_delta_jiffies;
                }
        }

        /*
         * Do not stop the tick, if we are only one off (or less)
         * or if the cpu is required for RCU:
         */
        if (!ts->tick_stopped && delta_jiffies <= 1)
                goto out;

        /* Schedule the tick, if we are at least one jiffie off */
        if ((long)delta_jiffies >= 1) {

                /*
                 * If this cpu is the one which updates jiffies, then
                 * give up the assignment and let it be taken by the
                 * cpu which runs the tick timer next, which might be
                 * this cpu as well. If we don't drop this here the
                 * jiffies might be stale and do_timer() never
                 * invoked. Keep track of the fact that it was the one
                 * which had the do_timer() duty last. If this cpu is
                 * the one which had the do_timer() duty last, we
                 * limit the sleep time to the timekeeping
                 * max_deferement value which we retrieved
                 * above. Otherwise we can sleep as long as we want.
                 */
                if (cpu == tick_do_timer_cpu) {
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
                        ts->do_timer_last = 1;
                } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
                        time_delta = KTIME_MAX;
                        ts->do_timer_last = 0;
                } else if (!ts->do_timer_last) {
                        time_delta = KTIME_MAX;
                }

#ifdef CONFIG_NO_HZ_FULL
                if (!ts->inidle) {
                        time_delta = min(time_delta,
                                         scheduler_tick_max_deferment());
                }
#endif

                /*
                 * calculate the expiry time for the next timer wheel
                 * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals
                 * that there is no timer pending or at least extremely
                 * far into the future (12 days for HZ=1000). In this
                 * case we set the expiry to the end of time.
                 */
                if (likely(delta_jiffies < NEXT_TIMER_MAX_DELTA)) {
                        /*
                         * Calculate the time delta for the next timer event.
                         * If the time delta exceeds the maximum time delta
                         * permitted by the current clocksource then adjust
                         * the time delta accordingly to ensure the
                         * clocksource does not wrap.
                         */
                        time_delta = min_t(u64, time_delta,
                                           tick_period.tv64 * delta_jiffies);
                }

                if (time_delta < KTIME_MAX)
                        expires = ktime_add_ns(last_update, time_delta);
                else
                        expires.tv64 = KTIME_MAX;

                /* Skip reprogram of event if its not changed */
                if (ts->tick_stopped && ktime_equal(expires, dev->next_event))
                        goto out;

                ret = expires;

                /*
                 * nohz_stop_sched_tick can be called several times before
                 * the nohz_restart_sched_tick is called. This happens when
                 * interrupts arrive which do not cause a reschedule. In the
                 * first call we save the current tick time, so we can restart
                 * the scheduler tick in nohz_restart_sched_tick.
                 */
                if (!ts->tick_stopped) {
                        nohz_balance_enter_idle(cpu);
                        calc_load_enter_idle();

                        ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
                        ts->tick_stopped = 1;
                        trace_tick_stop(1, " ");
                }

                /*
                 * If the expiration time == KTIME_MAX, then
                 * in this case we simply stop the tick timer.
                 */
                 if (unlikely(expires.tv64 == KTIME_MAX)) {
                        if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
                                hrtimer_cancel(&ts->sched_timer);
                        goto out;
                }

                if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
                        hrtimer_start(&ts->sched_timer, expires,
                                      HRTIMER_MODE_ABS_PINNED);
                        /* Check, if the timer was already in the past */
                        if (hrtimer_active(&ts->sched_timer))
                                goto out;
                } else if (!tick_program_event(expires, 0))
                                goto out;
                /*
                 * We are past the event already. So we crossed a
                 * jiffie boundary. Update jiffies and raise the
                 * softirq.
                 */
                tick_do_update_jiffies64(ktime_get());
        }
        raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
        ts->next_jiffies = next_jiffies;
        ts->last_jiffies = last_jiffies;
        ts->sleep_length = ktime_sub(dev->next_event, now);

        return ret;
}

static void tick_nohz_full_stop_tick(struct tick_sched *ts)
{
#ifdef CONFIG_NO_HZ_FULL
        int cpu = smp_processor_id();

        if (!tick_nohz_full_cpu(cpu) || is_idle_task(current))
                return;

        if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
                return;

        if (!can_stop_full_tick())
                return;

        tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
#endif
}

static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
{
        /*
         * If this cpu is offline and it is the one which updates
         * jiffies, then give up the assignment and let it be taken by
         * the cpu which runs the tick timer next. If we don't drop
         * this here the jiffies might be stale and do_timer() never
         * invoked.
         */
        if (unlikely(!cpu_online(cpu))) {
                if (cpu == tick_do_timer_cpu)
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
                return false;
        }

        if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
                ts->sleep_length = (ktime_t) { .tv64 = NSEC_PER_SEC/HZ };
                return false;
        }

        if (need_resched())
                return false;

        if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
                static int ratelimit;

                if (ratelimit < 10 &&
                    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
                        pr_warn("NOHZ: local_softirq_pending %02x\n",
                                (unsigned int) local_softirq_pending());
                        ratelimit++;
                }
                return false;
        }

        if (tick_nohz_full_enabled()) {
                /*
                 * Keep the tick alive to guarantee timekeeping progression
                 * if there are full dynticks CPUs around
                 */
                if (tick_do_timer_cpu == cpu)
                        return false;
                /*
                 * Boot safety: make sure the timekeeping duty has been
                 * assigned before entering dyntick-idle mode,
                 */
                if (tick_do_timer_cpu == TICK_DO_TIMER_NONE)
                        return false;
        }

        return true;
}

static void __tick_nohz_idle_enter(struct tick_sched *ts)
{
        ktime_t now, expires;
        int cpu = smp_processor_id();

        now = tick_nohz_start_idle(ts);

        if (can_stop_idle_tick(cpu, ts)) {
                int was_stopped = ts->tick_stopped;

                ts->idle_calls++;

                expires = tick_nohz_stop_sched_tick(ts, now, cpu);
                if (expires.tv64 > 0LL) {
                        ts->idle_sleeps++;
                        ts->idle_expires = expires;
                }

                if (!was_stopped && ts->tick_stopped)
                        ts->idle_jiffies = ts->last_jiffies;
        }
}

/**
 * tick_nohz_idle_enter - stop the idle tick from the idle task
 *
 * When the next event is more than a tick into the future, stop the idle tick
 * Called when we start the idle loop.
 *
 * The arch is responsible of calling:
 *
 * - rcu_idle_enter() after its last use of RCU before the CPU is put
 *  to sleep.
 * - rcu_idle_exit() before the first use of RCU after the CPU is woken up.
 */
void tick_nohz_idle_enter(void)
{
        struct tick_sched *ts;

        WARN_ON_ONCE(irqs_disabled());

        /*
         * Update the idle state in the scheduler domain hierarchy
         * when tick_nohz_stop_sched_tick() is called from the idle loop.
         * State will be updated to busy during the first busy tick after
         * exiting idle.
         */
        set_cpu_sd_state_idle();

        local_irq_disable();

        ts = &__get_cpu_var(tick_cpu_sched);
        ts->inidle = 1;
        __tick_nohz_idle_enter(ts);

        local_irq_enable();
}

/**
 * tick_nohz_irq_exit - update next tick event from interrupt exit
 *
 * When an interrupt fires while we are idle and it doesn't cause
 * a reschedule, it may still add, modify or delete a timer, enqueue
 * an RCU callback, etc...
 * So we need to re-calculate and reprogram the next tick event.
 */
void tick_nohz_irq_exit(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        if (ts->inidle)
                __tick_nohz_idle_enter(ts);
        else
                tick_nohz_full_stop_tick(ts);
}

/**
 * tick_nohz_get_sleep_length - return the length of the current sleep
 *
 * Called from power state control code with interrupts disabled
 */
ktime_t tick_nohz_get_sleep_length(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        return ts->sleep_length;
}

static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
        hrtimer_cancel(&ts->sched_timer);
        hrtimer_set_expires(&ts->sched_timer, ts->last_tick);

        while (1) {
                /* Forward the time to expire in the future */
                hrtimer_forward(&ts->sched_timer, now, tick_period);

                if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
                        hrtimer_start_expires(&ts->sched_timer,
                                              HRTIMER_MODE_ABS_PINNED);
                        /* Check, if the timer was already in the past */
                        if (hrtimer_active(&ts->sched_timer))
                                break;
                } else {
                        if (!tick_program_event(
                                hrtimer_get_expires(&ts->sched_timer), 0))
                                break;
                }
                /* Reread time and update jiffies */
                now = ktime_get();
                tick_do_update_jiffies64(now);
        }
}

static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
{
        /* Update jiffies first */
        tick_do_update_jiffies64(now);
        update_cpu_load_nohz();

        calc_load_exit_idle();
        touch_softlockup_watchdog();
        /*
         * Cancel the scheduled timer and restore the tick
         */
        ts->tick_stopped  = 0;
        ts->idle_exittime = now;

        tick_nohz_restart(ts, now);
}

static void tick_nohz_account_idle_ticks(struct tick_sched *ts)
{
#ifndef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
        unsigned long ticks;

        if (vtime_accounting_enabled())
                return;
        /*
         * We stopped the tick in idle. Update process times would miss the
         * time we slept as update_process_times does only a 1 tick
         * accounting. Enforce that this is accounted to idle !
         */
        ticks = jiffies - ts->idle_jiffies;
        /*
         * We might be one off. Do not randomly account a huge number of ticks!
         */
        if (ticks && ticks < LONG_MAX)
                account_idle_ticks(ticks);
#endif
}

/**
 * tick_nohz_idle_exit - restart the idle tick from the idle task
 *
 * Restart the idle tick when the CPU is woken up from idle
 * This also exit the RCU extended quiescent state. The CPU
 * can use RCU again after this function is called.
 */
void tick_nohz_idle_exit(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
        ktime_t now;

        local_irq_disable();

        WARN_ON_ONCE(!ts->inidle);

        ts->inidle = 0;

        if (ts->idle_active || ts->tick_stopped)
                now = ktime_get();

        if (ts->idle_active)
                tick_nohz_stop_idle(ts, now);

        if (ts->tick_stopped) {
                tick_nohz_restart_sched_tick(ts, now);
                tick_nohz_account_idle_ticks(ts);
        }

        local_irq_enable();
}

static int tick_nohz_reprogram(struct tick_sched *ts, ktime_t now)
{
        hrtimer_forward(&ts->sched_timer, now, tick_period);
        return tick_program_event(hrtimer_get_expires(&ts->sched_timer), 0);
}

/*
 * The nohz low res interrupt handler
 */
static void tick_nohz_handler(struct clock_event_device *dev)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
        struct pt_regs *regs = get_irq_regs();
        ktime_t now = ktime_get();

        dev->next_event.tv64 = KTIME_MAX;

        tick_sched_do_timer(now);
        tick_sched_handle(ts, regs);

        while (tick_nohz_reprogram(ts, now)) {
                now = ktime_get();
                tick_do_update_jiffies64(now);
        }
}

/**
 * tick_nohz_switch_to_nohz - switch to nohz mode
 */
static void tick_nohz_switch_to_nohz(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
        ktime_t next;

        if (!tick_nohz_enabled)
                return;

        local_irq_disable();
        if (tick_switch_to_oneshot(tick_nohz_handler)) {
                local_irq_enable();
                return;
        }
        tick_nohz_active = 1;
        ts->nohz_mode = NOHZ_MODE_LOWRES;

        /*
         * Recycle the hrtimer in ts, so we can share the
         * hrtimer_forward with the highres code.
         */
        hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
        /* Get the next period */
        next = tick_init_jiffy_update();

        for (;;) {
                hrtimer_set_expires(&ts->sched_timer, next);
                if (!tick_program_event(next, 0))
                        break;
                next = ktime_add(next, tick_period);
        }
        local_irq_enable();
}

/*
 * When NOHZ is enabled and the tick is stopped, we need to kick the
 * tick timer from irq_enter() so that the jiffies update is kept
 * alive during long running softirqs. That's ugly as hell, but
 * correctness is key even if we need to fix the offending softirq in
 * the first place.
 *
 * Note, this is different to tick_nohz_restart. We just kick the
 * timer and do not touch the other magic bits which need to be done
 * when idle is left.
 */
static void tick_nohz_kick_tick(struct tick_sched *ts, ktime_t now)
{
#if 0
        /* Switch back to 2.6.27 behaviour */
        ktime_t delta;

        /*
         * Do not touch the tick device, when the next expiry is either
         * already reached or less/equal than the tick period.
         */
        delta = ktime_sub(hrtimer_get_expires(&ts->sched_timer), now);
        if (delta.tv64 <= tick_period.tv64)
                return;

        tick_nohz_restart(ts, now);
#endif
}

static inline void tick_nohz_irq_enter(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
        ktime_t now;

        if (!ts->idle_active && !ts->tick_stopped)
                return;
        now = ktime_get();
        if (ts->idle_active)
                tick_nohz_stop_idle(ts, now);
        if (ts->tick_stopped) {
                tick_nohz_update_jiffies(now);
                tick_nohz_kick_tick(ts, now);
        }
}

#else

static inline void tick_nohz_switch_to_nohz(void) { }
static inline void tick_nohz_irq_enter(void) { }

#endif /* CONFIG_NO_HZ_COMMON */

/*
 * Called from irq_enter to notify about the possible interruption of idle()
 */
void tick_irq_enter(void)
{
        tick_check_oneshot_broadcast_this_cpu();
        tick_nohz_irq_enter();
}

/*
 * High resolution timer specific code
 */
#ifdef CONFIG_HIGH_RES_TIMERS
/*
 * We rearm the timer until we get disabled by the idle code.
 * Called with interrupts disabled.
 */
static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
{
        struct tick_sched *ts =
                container_of(timer, struct tick_sched, sched_timer);
        struct pt_regs *regs = get_irq_regs();
        ktime_t now = ktime_get();

        tick_sched_do_timer(now);

        /*
         * Do not call, when we are not in irq context and have
         * no valid regs pointer
         */
        if (regs)
                tick_sched_handle(ts, regs);

        hrtimer_forward(timer, now, tick_period);

        return HRTIMER_RESTART;
}

static int sched_skew_tick;

static int __init skew_tick(char *str)
{
        get_option(&str, &sched_skew_tick);

        return 0;
}
early_param("skew_tick", skew_tick);

/**
 * tick_setup_sched_timer - setup the tick emulation timer
 */
void tick_setup_sched_timer(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
        ktime_t now = ktime_get();

        /*
         * Emulate tick processing via per-CPU hrtimers:
         */
        hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
        ts->sched_timer.function = tick_sched_timer;

        /* Get the next period (per cpu) */
        hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());

        /* Offset the tick to avert jiffies_lock contention. */
        if (sched_skew_tick) {
                u64 offset = ktime_to_ns(tick_period) >> 1;
                do_div(offset, num_possible_cpus());
                offset *= smp_processor_id();
                hrtimer_add_expires_ns(&ts->sched_timer, offset);
        }

        for (;;) {
                hrtimer_forward(&ts->sched_timer, now, tick_period);
                hrtimer_start_expires(&ts->sched_timer,
                                      HRTIMER_MODE_ABS_PINNED);
                /* Check, if the timer was already in the past */
                if (hrtimer_active(&ts->sched_timer))
                        break;
                now = ktime_get();
        }

#ifdef CONFIG_NO_HZ_COMMON
        if (tick_nohz_enabled) {
                ts->nohz_mode = NOHZ_MODE_HIGHRES;
                tick_nohz_active = 1;
        }
#endif
}
#endif /* HIGH_RES_TIMERS */

#if defined CONFIG_NO_HZ_COMMON || defined CONFIG_HIGH_RES_TIMERS
void tick_cancel_sched_timer(int cpu)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

# ifdef CONFIG_HIGH_RES_TIMERS
        if (ts->sched_timer.base)
                hrtimer_cancel(&ts->sched_timer);
# endif

        memset(ts, 0, sizeof(*ts));
}
#endif

/**
 * Async notification about clocksource changes
 */
void tick_clock_notify(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                set_bit(0, &per_cpu(tick_cpu_sched, cpu).check_clocks);
}

/*
 * Async notification about clock event changes
 */
void tick_oneshot_notify(void)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        set_bit(0, &ts->check_clocks);
}

/**
 * Check, if a change happened, which makes oneshot possible.
 *
 * Called cyclic from the hrtimer softirq (driven by the timer
 * softirq) allow_nohz signals, that we can switch into low-res nohz
 * mode, because high resolution timers are disabled (either compile
 * or runtime).
 */
int tick_check_oneshot_change(int allow_nohz)
{
        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        if (!test_and_clear_bit(0, &ts->check_clocks))
                return 0;

        if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
                return 0;

        if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
                return 0;

        if (!allow_nohz)
                return 1;

        tick_nohz_switch_to_nohz();
        return 0;
}