tick-sched.c

linux 2.6.19 kernel source code before patching

/*
 *  linux/kernel/time/tick-sched.c
 *
 *  Copyright(C) 2005-2006, Thomas Gleixner <[e-mail]>
 *  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 <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;

	/* 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();
	if (unlikely(local_softirq_pending())) {
		static int ratelimit;

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

	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;

	if (rcu_needs_cpu(cpu))
		delta_jiffies = 1;
	/*
	 * 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) {

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

			ts->idle_tick = ts->sched_timer.expires;
			ts->tick_stopped = 1;
			ts->idle_jiffies = last_jiffies;
		}

		/*
		 * 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 = -1;

		ts->idle_sleeps++;

		/*
		 * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
		 * there is no timer pending or at least extremly far
		 * into the future (12 days for HZ=1000). In this case
		 * we simply stop the tick timer:
		 */
		if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
			ts->idle_expires.tv64 = KTIME_MAX;
			if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
				hrtimer_cancel(&ts->sched_timer);
			goto out;
		}

		/*
		 * 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;

		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();
	select_nohz_load_balancer(0);
	tick_do_update_jiffies64(now);
	cpu_clear(cpu, nohz_cpu_mask);