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
 *
 *  For licencing details see kernel-base/COPYING
 */
#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 "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;

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

	/* 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);
	}
	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.
 */
void tick_nohz_update_jiffies(void)
{
	int cpu = smp_processor_id();
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
	unsigned long flags;
	ktime_t now;

	if (!ts->tick_stopped)
		return;

	cpu_clear(cpu, nohz_cpu_mask);
	now = ktime_get();

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

/**
 * 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(void)
{
	unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
	struct tick_sched *ts;
	ktime_t last_update, expires, now, delta;
	int cpu;

	local_irq_save(flags);

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

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

	if (need_resched())
		goto end;

	cpu = smp_processor_id();
	BUG_ON(local_softirq_pending());

	now = ktime_get();
	/*
	 * When called from irq_exit we need to account the idle sleep time
	 * correctly.
	 */
	if (ts->tick_stopped) {
		delta = ktime_sub(now, ts->idle_entrytime);
		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
	}

	ts->idle_entrytime = now;
	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;
	} while (read_seqretry(&xtime_lock, seq));

	/* 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 || rcu_needs_cpu(cpu)))
		goto out;

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

		if (rcu_needs_cpu(cpu))
			delta_jiffies = 1;
		else
			cpu_set(cpu, nohz_cpu_mask);
		/*
		 * 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) {
			ts->idle_tick = ts->sched_timer.expires;
			ts->tick_stopped = 1;
			ts->idle_jiffies = last_jiffies;
		}
		/*
		 * calculate the expiry time for the next timer wheel
		 * timer
		 */
		expires = ktime_add_ns(last_update, tick_period.tv64 *
				       delta_jiffies);
		ts->idle_expires = expires;
		ts->idle_sleeps++;

		if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
			hrtimer_start(&ts->sched_timer, expires,
				      HRTIMER_MODE_ABS);
			/* 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());
		cpu_clear(cpu, nohz_cpu_mask);
	}
	raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
	ts->next_jiffies = next_jiffies;
	ts->last_jiffies = last_jiffies;
end:
	local_irq_restore(flags);
}

/**
 * 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);
	unsigned long ticks;
	ktime_t now, delta;

	if (!ts->tick_stopped)
		return;

	/* Update jiffies first */
	now = ktime_get();

	local_irq_disable();
	tick_do_update_jiffies64(now);
	cpu_clear(cpu, nohz_cpu_mask);

	/* Account the idle time */
	delta = ktime_sub(now, ts->idle_entrytime);
	ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);

	/*
	 * 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) {
		add_preempt_count(HARDIRQ_OFFSET);
		account_system_time(current, HARDIRQ_OFFSET,
				    jiffies_to_cputime(ticks));
		sub_preempt_count(HARDIRQ_OFFSET);
	}

	/*
	 * Cancel the scheduled timer and restore the tick
	 */
	ts->tick_stopped  = 0;
	hrtimer_cancel(&ts->sched_timer);
	ts->sched_timer.expires = 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(&ts->sched_timer,
				      ts->sched_timer.expires,
				      HRTIMER_MODE_ABS);
			/* Check, if the timer was already in the past */
			if (hrtimer_active(&ts->sched_timer))
				break;
		} else {
			if (!tick_program_event(ts->sched_timer.expires, 0))
				break;
		}
		/* Update jiffies and reread time */
		tick_do_update_jiffies64(now);
		now = ktime_get();
	}
	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(ts->sched_timer.expires, 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;

	/* Check, if the jiffies need an update */
	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);

	/* Do not restart, when we are in the idle loop */
	if (ts->tick_stopped)
		return;

	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 (;;) {
		ts->sched_timer.expires = 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());
}

#else

static inline void tick_nohz_switch_to_nohz(void) { }

#endif /* NO_HZ */

/*
 * 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 hrtimer_cpu_base *base = timer->base->cpu_base;
	struct pt_regs *regs = get_irq_regs();
	ktime_t now = ktime_get();

	/* Check, if the jiffies need an update */
	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() might take tasklist_lock, hence
		 * drop the base lock. sched-tick hrtimers are per-CPU and
		 * never accessible by userspace APIs, so this is safe to do.
		 */
		spin_unlock(&base->lock);
		update_process_times(user_mode(regs));
		profile_tick(CPU_PROFILING);
		spin_lock(&base->lock);
	}

	/* Do not restart, when we are in the idle loop */
	if (ts->tick_stopped)
		return HRTIMER_NORESTART;

	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();

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

	/* Get the next period */
	ts->sched_timer.expires = tick_init_jiffy_update();

	for (;;) {
		hrtimer_forward(&ts->sched_timer, now, tick_period);
		hrtimer_start(&ts->sched_timer, ts->sched_timer.expires,
			      HRTIMER_MODE_ABS);
		/* 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
}

void tick_cancel_sched_timer(int cpu)
{
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

	if (ts->sched_timer.base)
		hrtimer_cancel(&ts->sched_timer);
	ts->tick_stopped = 0;
	ts->nohz_mode = NOHZ_MODE_INACTIVE;
}
#endif /* HIGH_RES_TIMERS */

/**
 * 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_is_continuous() || !tick_is_oneshot_available())
		return 0;

	if (!allow_nohz)
		return 1;

	tick_nohz_switch_to_nohz();
	return 0;
}