clockevents: Set noop handler in clockevents_exchange_device()

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.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/tick.h>
#include <linux/module.h>

#include <asm/irq_regs.h>

#include "tick-internal.h"

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

/*
 * The time, when the last jiffy update happened. Protected by xtime_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 xtime_lock:
         */
        delta = ktime_sub(now, last_jiffies_update);
        if (delta.tv64 < tick_period.tv64)
                return;

        /* Reevalute with xtime_lock held */
        write_seqlock(&xtime_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);
        }
        write_sequnlock(&xtime_lock);
}

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

        write_seqlock(&xtime_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(&xtime_lock);
        return period;
}

/*
 * NOHZ - aka dynamic tick functionality
 */
#ifdef CONFIG_NO_HZ
/*
 * NO HZ enabled ?
 */
static int tick_nohz_enabled __read_mostly  = 1;

/*
 * 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)
{
        int cpu = smp_processor_id();
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        unsigned long flags;

        cpumask_clear_cpu(cpu, nohz_cpu_mask);
        ts->idle_waketime = now;

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

        touch_softlockup_watchdog();
}

static void tick_nohz_stop_idle(int cpu, ktime_t now)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        ktime_t delta;

        delta = ktime_sub(now, ts->idle_entrytime);
        ts->idle_lastupdate = now;
        ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
        ts->idle_active = 0;

        sched_clock_idle_wakeup_event(0);
}

static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
{
        ktime_t now, delta;

        now = ktime_get();
        if (ts->idle_active) {
                delta = ktime_sub(now, ts->idle_entrytime);
                ts->idle_lastupdate = now;
                ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
        }
        ts->idle_entrytime = now;
        ts->idle_active = 1;
        sched_clock_idle_sleep_event();
        return now;
}

u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

        if (!tick_nohz_enabled)
                return -1;

        if (ts->idle_active)
                *last_update_time = ktime_to_us(ts->idle_lastupdate);
        else
                *last_update_time = ktime_to_us(ktime_get());

        return ktime_to_us(ts->idle_sleeptime);
}
EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);

/**
 * tick_nohz_stop_sched_tick - 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 either from the idle loop or from irq_exit() when an idle period was
 * just interrupted by an interrupt which did not cause a reschedule.
 */
void tick_nohz_stop_sched_tick(int inidle)
{
        unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
        struct tick_sched *ts;
        ktime_t last_update, expires, now;
        struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
        u64 time_delta;
        int cpu;

        local_irq_save(flags);

        cpu = smp_processor_id();
        ts = &per_cpu(tick_cpu_sched, cpu);

        /*
         * Call to tick_nohz_start_idle stops the last_update_time from being
         * updated. Thus, it must not be called in the event we are called from
         * irq_exit() with the prior state different than idle.
         */
        if (!inidle && !ts->inidle)
                goto end;

        /*
         * Set ts->inidle unconditionally. Even if the system did not
         * switch to NOHZ mode the cpu frequency governers rely on the
         * update of the idle time accounting in tick_nohz_start_idle().
         */
        ts->inidle = 1;

        now = tick_nohz_start_idle(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;
        }

        if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
                goto end;

        if (need_resched())
                goto end;

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

                if (ratelimit < 10) {
                        printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
                               local_softirq_pending());
                        ratelimit++;
                }
                goto end;
        }

        ts->idle_calls++;
        /* Read jiffies and the time when jiffies were updated last */
        do {
                seq = read_seqbegin(&xtime_lock);
                last_update = last_jiffies_update;
                last_jiffies = jiffies;

                /*
                 * On SMP we really should only care for the CPU which
                 * has the do_timer duty assigned. All other CPUs can
                 * sleep as long as they want.
                 */
                if (cpu == tick_do_timer_cpu ||
                    tick_do_timer_cpu == TICK_DO_TIMER_NONE)
                        time_delta = timekeeping_max_deferment();
                else
                        time_delta = KTIME_MAX;
        } while (read_seqretry(&xtime_lock, seq));

        if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
            arch_needs_cpu(cpu)) {
                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;
        }
        /*
         * Do not stop the tick, if we are only one off
         * 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) {

                /*
                 * 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);
                        expires = ktime_add_ns(last_update, time_delta);
                } else {
                        expires.tv64 = KTIME_MAX;
                }

                /*
                 * 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.
                 */
                if (cpu == tick_do_timer_cpu)
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;

                if (delta_jiffies > 1)
                        cpumask_set_cpu(cpu, nohz_cpu_mask);

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

                /*
                 * 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) {
                        if (select_nohz_load_balancer(1)) {
                                /*
                                 * sched tick not stopped!
                                 */
                                cpumask_clear_cpu(cpu, nohz_cpu_mask);
                                goto out;
                        }

                        ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
                        ts->tick_stopped = 1;
                        ts->idle_jiffies = last_jiffies;
                        rcu_enter_nohz();
                }

                ts->idle_sleeps++;

                /* Mark expires */
                ts->idle_expires = expires;

                /*
                 * 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());
                cpumask_clear_cpu(cpu, nohz_cpu_mask);
        }
        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);
end:
        local_irq_restore(flags);
}

/**
 * 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->idle_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;
                }
                /* Update jiffies and reread time */
                tick_do_update_jiffies64(now);
                now = ktime_get();
        }
}

/**
 * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
 *
 * Restart the idle tick when the CPU is woken up from idle
 */
void tick_nohz_restart_sched_tick(void)
{
        int cpu = smp_processor_id();
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
        unsigned long ticks;
#endif
        ktime_t now;

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

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

        if (!ts->inidle || !ts->tick_stopped) {
                ts->inidle = 0;
                local_irq_enable();
                return;
        }

        ts->inidle = 0;

        rcu_exit_nohz();

        /* Update jiffies first */
        select_nohz_load_balancer(0);
        tick_do_update_jiffies64(now);
        cpumask_clear_cpu(cpu, nohz_cpu_mask);

#ifndef CONFIG_VIRT_CPU_ACCOUNTING
        /*
         * 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

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

        tick_nohz_restart(ts, now);

        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();
        int cpu = smp_processor_id();
        ktime_t now = ktime_get();

        dev->next_event.tv64 = KTIME_MAX;

        /*
         * 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
         * xtime_lock.
         */
        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
                tick_do_timer_cpu = cpu;

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

        /*
         * 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();
                ts->idle_jiffies++;
        }

        update_process_times(user_mode(regs));
        profile_tick(CPU_PROFILING);

        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;
        }

        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();

        printk(KERN_INFO "Switched to NOHz mode on CPU #%d\n",
               smp_processor_id());
}

/*
 * 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(int cpu, ktime_t now)
{
#if 0
        /* Switch back to 2.6.27 behaviour */

        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        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_check_nohz(int cpu)
{
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
        ktime_t now;

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

#else

static inline void tick_nohz_switch_to_nohz(void) { }
static inline void tick_check_nohz(int cpu) { }

#endif /* NO_HZ */

/*
 * Called from irq_enter to notify about the possible interruption of idle()
 */
void tick_check_idle(int cpu)
{
        tick_check_oneshot_broadcast(cpu);
        tick_check_nohz(cpu);
}

/*
 * 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 and timer->base->cpu_base->lock held.
 */
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();
        int cpu = smp_processor_id();

#ifdef CONFIG_NO_HZ
        /*
         * 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
         * xtime_lock.
         */
        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
                tick_do_timer_cpu = cpu;
#endif

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

        /*
         * Do not call, when we are not in irq context and have
         * no valid regs pointer
         */
        if (regs) {
                /*
                 * 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();
                        ts->idle_jiffies++;
                }
                update_process_times(user_mode(regs));
                profile_tick(CPU_PROFILING);
        }

        hrtimer_forward(timer, now, tick_period);

        return HRTIMER_RESTART;
}

/**
 * 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();
        u64 offset;

        /*
         * 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 = 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
        if (tick_nohz_enabled)
                ts->nohz_mode = NOHZ_MODE_HIGHRES;
#endif
}
#endif /* HIGH_RES_TIMERS */

#if defined CONFIG_NO_HZ || 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

        ts->nohz_mode = NOHZ_MODE_INACTIVE;
}
#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;
}