tick-sched.c 31.8 KB
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/*
 *  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
 *
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 *  Distribute under GPLv2.
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 */
#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>
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#include <linux/nmi.h>
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#include <linux/profile.h>
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#include <linux/sched/signal.h>
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#include <linux/sched/clock.h>
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#include <linux/sched/stat.h>
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#include <linux/sched/nohz.h>
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#include <linux/module.h>
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#include <linux/irq_work.h>
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#include <linux/posix-timers.h>
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#include <linux/context_tracking.h>
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#include <asm/irq_regs.h>

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#include "tick-internal.h"

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#include <trace/events/timer.h>

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/*
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 * Per-CPU nohz control structure
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 */
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static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
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struct tick_sched *tick_get_tick_sched(int cpu)
{
	return &per_cpu(tick_cpu_sched, cpu);
}

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#if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS)
/*
 * The time, when the last jiffy update happened. Protected by jiffies_lock.
 */
static ktime_t last_jiffies_update;

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

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

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	/* Reevaluate with jiffies_lock held */
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	write_seqlock(&jiffies_lock);
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	delta = ktime_sub(now, last_jiffies_update);
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	if (delta >= tick_period) {
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		delta = ktime_sub(delta, tick_period);
		last_jiffies_update = ktime_add(last_jiffies_update,
						tick_period);

		/* Slow path for long timeouts */
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		if (unlikely(delta >= tick_period)) {
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			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);
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		/* Keep the tick_next_period variable up to date */
		tick_next_period = ktime_add(last_jiffies_update, tick_period);
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	} else {
		write_sequnlock(&jiffies_lock);
		return;
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	}
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	write_sequnlock(&jiffies_lock);
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	update_wall_time();
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}

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

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

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

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

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

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static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
{
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#ifdef CONFIG_NO_HZ_COMMON
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	/*
	 * 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) {
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		touch_softlockup_watchdog_sched();
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		if (is_idle_task(current))
			ts->idle_jiffies++;
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		/*
		 * In case the current tick fired too early past its expected
		 * expiration, make sure we don't bypass the next clock reprogramming
		 * to the same deadline.
		 */
		ts->next_tick = 0;
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	}
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#endif
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	update_root_process_times(regs);
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	profile_tick(CPU_PROFILING);
}
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#endif
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#ifdef CONFIG_NO_HZ_FULL
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cpumask_var_t tick_nohz_full_mask;
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cpumask_var_t housekeeping_mask;
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bool tick_nohz_full_running;
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static atomic_t tick_dep_mask;
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static bool check_tick_dependency(atomic_t *dep)
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{
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	int val = atomic_read(dep);

	if (val & TICK_DEP_MASK_POSIX_TIMER) {
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		trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);
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		return true;
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	}

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	if (val & TICK_DEP_MASK_PERF_EVENTS) {
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		trace_tick_stop(0, TICK_DEP_MASK_PERF_EVENTS);
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		return true;
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	}

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	if (val & TICK_DEP_MASK_SCHED) {
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		trace_tick_stop(0, TICK_DEP_MASK_SCHED);
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		return true;
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	}

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	if (val & TICK_DEP_MASK_CLOCK_UNSTABLE) {
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		trace_tick_stop(0, TICK_DEP_MASK_CLOCK_UNSTABLE);
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		return true;
	}

	return false;
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}

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static bool can_stop_full_tick(int cpu, struct tick_sched *ts)
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{
	WARN_ON_ONCE(!irqs_disabled());

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	if (unlikely(!cpu_online(cpu)))
		return false;

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	if (check_tick_dependency(&tick_dep_mask))
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		return false;

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	if (check_tick_dependency(&ts->tick_dep_mask))
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		return false;

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	if (check_tick_dependency(&current->tick_dep_mask))
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		return false;

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	if (check_tick_dependency(&current->signal->tick_dep_mask))
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		return false;

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	return true;
}

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static void nohz_full_kick_func(struct irq_work *work)
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{
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	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
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}

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

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/*
 * Kick this CPU if it's full dynticks in order to force it to
 * re-evaluate its dependency on the tick and restart it if necessary.
 * This kick, unlike tick_nohz_full_kick_cpu() and tick_nohz_full_kick_all(),
 * is NMI safe.
 */
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static void tick_nohz_full_kick(void)
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{
	if (!tick_nohz_full_cpu(smp_processor_id()))
		return;

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	irq_work_queue(this_cpu_ptr(&nohz_full_kick_work));
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}

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/*
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 * Kick the CPU if it's full dynticks in order to force it to
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 * re-evaluate its dependency on the tick and restart it if necessary.
 */
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void tick_nohz_full_kick_cpu(int cpu)
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{
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	if (!tick_nohz_full_cpu(cpu))
		return;

	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
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}

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

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	if (!tick_nohz_full_running)
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		return;

	preempt_disable();
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	for_each_cpu_and(cpu, tick_nohz_full_mask, cpu_online_mask)
		tick_nohz_full_kick_cpu(cpu);
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	preempt_enable();
}

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static void tick_nohz_dep_set_all(atomic_t *dep,
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				  enum tick_dep_bits bit)
{
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	int prev;
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	prev = atomic_fetch_or(BIT(bit), dep);
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	if (!prev)
		tick_nohz_full_kick_all();
}

/*
 * Set a global tick dependency. Used by perf events that rely on freq and
 * by unstable clock.
 */
void tick_nohz_dep_set(enum tick_dep_bits bit)
{
	tick_nohz_dep_set_all(&tick_dep_mask, bit);
}

void tick_nohz_dep_clear(enum tick_dep_bits bit)
{
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	atomic_andnot(BIT(bit), &tick_dep_mask);
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}

/*
 * Set per-CPU tick dependency. Used by scheduler and perf events in order to
 * manage events throttling.
 */
void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
{
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	int prev;
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	struct tick_sched *ts;

	ts = per_cpu_ptr(&tick_cpu_sched, cpu);

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	prev = atomic_fetch_or(BIT(bit), &ts->tick_dep_mask);
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	if (!prev) {
		preempt_disable();
		/* Perf needs local kick that is NMI safe */
		if (cpu == smp_processor_id()) {
			tick_nohz_full_kick();
		} else {
			/* Remote irq work not NMI-safe */
			if (!WARN_ON_ONCE(in_nmi()))
				tick_nohz_full_kick_cpu(cpu);
		}
		preempt_enable();
	}
}

void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
{
	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);

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	atomic_andnot(BIT(bit), &ts->tick_dep_mask);
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}

/*
 * Set a per-task tick dependency. Posix CPU timers need this in order to elapse
 * per task timers.
 */
void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
{
	/*
	 * We could optimize this with just kicking the target running the task
	 * if that noise matters for nohz full users.
	 */
	tick_nohz_dep_set_all(&tsk->tick_dep_mask, bit);
}

void tick_nohz_dep_clear_task(struct task_struct *tsk, enum tick_dep_bits bit)
{
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	atomic_andnot(BIT(bit), &tsk->tick_dep_mask);
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}

/*
 * Set a per-taskgroup tick dependency. Posix CPU timers need this in order to elapse
 * per process timers.
 */
void tick_nohz_dep_set_signal(struct signal_struct *sig, enum tick_dep_bits bit)
{
	tick_nohz_dep_set_all(&sig->tick_dep_mask, bit);
}

void tick_nohz_dep_clear_signal(struct signal_struct *sig, enum tick_dep_bits bit)
{
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	atomic_andnot(BIT(bit), &sig->tick_dep_mask);
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}

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

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

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	ts = this_cpu_ptr(&tick_cpu_sched);
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	if (ts->tick_stopped) {
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		if (atomic_read(&current->tick_dep_mask) ||
		    atomic_read(&current->signal->tick_dep_mask))
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			tick_nohz_full_kick();
	}
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out:
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	local_irq_restore(flags);
}

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/* Parse the boot-time nohz CPU list from the kernel parameters. */
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static int __init tick_nohz_full_setup(char *str)
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{
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	alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
	if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
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		pr_warn("NO_HZ: Incorrect nohz_full cpumask\n");
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		free_bootmem_cpumask_var(tick_nohz_full_mask);
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		return 1;
	}
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	tick_nohz_full_running = true;
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	return 1;
}
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__setup("nohz_full=", tick_nohz_full_setup);
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static int tick_nohz_cpu_down(unsigned int cpu)
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{
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	/*
	 * The boot CPU handles housekeeping duty (unbound timers,
	 * workqueues, timekeeping, ...) on behalf of full dynticks
	 * CPUs. It must remain online when nohz full is enabled.
	 */
	if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
		return -EBUSY;
	return 0;
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}

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

#ifdef CONFIG_NO_HZ_FULL_ALL
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	if (!alloc_cpumask_var(&tick_nohz_full_mask, GFP_KERNEL)) {
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		WARN(1, "NO_HZ: Can't allocate full dynticks cpumask\n");
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		return err;
	}
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	err = 0;
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	cpumask_setall(tick_nohz_full_mask);
	tick_nohz_full_running = true;
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#endif
	return err;
}

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void __init tick_nohz_init(void)
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{
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	int cpu, ret;
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	if (!tick_nohz_full_running) {
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		if (tick_nohz_init_all() < 0)
			return;
	}
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	if (!alloc_cpumask_var(&housekeeping_mask, GFP_KERNEL)) {
		WARN(1, "NO_HZ: Can't allocate not-full dynticks cpumask\n");
		cpumask_clear(tick_nohz_full_mask);
		tick_nohz_full_running = false;
		return;
	}

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	/*
	 * Full dynticks uses irq work to drive the tick rescheduling on safe
	 * locking contexts. But then we need irq work to raise its own
	 * interrupts to avoid circular dependency on the tick
	 */
	if (!arch_irq_work_has_interrupt()) {
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		pr_warn("NO_HZ: Can't run full dynticks because arch doesn't support irq work self-IPIs\n");
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		cpumask_clear(tick_nohz_full_mask);
		cpumask_copy(housekeeping_mask, cpu_possible_mask);
		tick_nohz_full_running = false;
		return;
	}

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	cpu = smp_processor_id();

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

	cpumask_andnot(housekeeping_mask,
		       cpu_possible_mask, tick_nohz_full_mask);

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	for_each_cpu(cpu, tick_nohz_full_mask)
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		context_tracking_cpu_set(cpu);

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	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
					"kernel/nohz:predown", NULL,
					tick_nohz_cpu_down);
	WARN_ON(ret < 0);
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	pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
		cpumask_pr_args(tick_nohz_full_mask));
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	/*
	 * We need at least one CPU to handle housekeeping work such
	 * as timekeeping, unbound timers, workqueues, ...
	 */
	WARN_ON_ONCE(cpumask_empty(housekeeping_mask));
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}
#endif

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/*
 * NOHZ - aka dynamic tick functionality
 */
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#ifdef CONFIG_NO_HZ_COMMON
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/*
 * NO HZ enabled ?
 */
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bool tick_nohz_enabled __read_mostly  = true;
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unsigned long tick_nohz_active  __read_mostly;
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/*
 * Enable / Disable tickless mode
 */
static int __init setup_tick_nohz(char *str)
{
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	return (kstrtobool(str, &tick_nohz_enabled) == 0);
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}

__setup("nohz=", setup_tick_nohz);

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int tick_nohz_tick_stopped(void)
{
	return __this_cpu_read(tick_cpu_sched.tick_stopped);
}

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/**
 * 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
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 * 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.
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 */
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static void tick_nohz_update_jiffies(ktime_t now)
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{
	unsigned long flags;

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	__this_cpu_write(tick_cpu_sched.idle_waketime, now);
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	local_irq_save(flags);
	tick_do_update_jiffies64(now);
	local_irq_restore(flags);
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	touch_softlockup_watchdog_sched();
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}

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/*
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 * Updates the per-CPU time idle statistics counters
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 */
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static void
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update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_update_time)
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{
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	ktime_t delta;
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	if (ts->idle_active) {
		delta = ktime_sub(now, ts->idle_entrytime);
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		if (nr_iowait_cpu(cpu) > 0)
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			ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
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		else
			ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
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		ts->idle_entrytime = now;
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	}
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	if (last_update_time)
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		*last_update_time = ktime_to_us(now);

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}

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

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static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
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{
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	ktime_t now = ktime_get();
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	ts->idle_entrytime = now;
	ts->idle_active = 1;
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	sched_clock_idle_sleep_event();
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	return now;
}

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/**
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 * get_cpu_idle_time_us - get the total idle time of a CPU
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 * @cpu: CPU number to query
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 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
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 *
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 * Return the cumulative idle time (since boot) for a given
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 * CPU, in microseconds.
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 *
 * 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.
 */
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u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
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	ktime_t now, idle;
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	if (!tick_nohz_active)
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		return -1;

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	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);
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}
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EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
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/**
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 * get_cpu_iowait_time_us - get the total iowait time of a CPU
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 * @cpu: CPU number to query
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 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
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 *
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 * Return the cumulative iowait time (since boot) for a given
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 * 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);
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	ktime_t now, iowait;
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	if (!tick_nohz_active)
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		return -1;

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	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);
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			iowait = ktime_add(ts->iowait_sleeptime, delta);
		} else {
			iowait = ts->iowait_sleeptime;
		}
	}
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	return ktime_to_us(iowait);
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}
EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);

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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);

	/* 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);
	else
		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
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	/*
	 * Reset to make sure next tick stop doesn't get fooled by past
	 * cached clock deadline.
	 */
	ts->next_tick = 0;
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}

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static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
					 ktime_t now, int cpu)
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{
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	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
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	u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
	unsigned long seq, basejiff;
	ktime_t	tick;
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	/* Read jiffies and the time when jiffies were updated last */
	do {
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		seq = read_seqbegin(&jiffies_lock);
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		basemono = last_jiffies_update;
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		basejiff = jiffies;
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	} while (read_seqretry(&jiffies_lock, seq));
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	ts->last_jiffies = basejiff;
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	if (rcu_needs_cpu(basemono, &next_rcu) ||
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	    arch_needs_cpu() || irq_work_needs_cpu()) {
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		next_tick = basemono + TICK_NSEC;
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	} else {
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		/*
		 * Get the next pending timer. If high resolution
		 * timers are enabled this only takes the timer wheel
		 * timers into account. If high resolution timers are
		 * disabled this also looks at the next expiring
		 * hrtimer.
		 */
		next_tmr = get_next_timer_interrupt(basejiff, basemono);
		ts->next_timer = next_tmr;
		/* Take the next rcu event into account */
		next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
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	}
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	/*
	 * If the tick is due in the next period, keep it ticking or
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	 * force prod the timer.
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	 */
	delta = next_tick - basemono;
	if (delta <= (u64)TICK_NSEC) {
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		/*
		 * Tell the timer code that the base is not idle, i.e. undo
		 * the effect of get_next_timer_interrupt():
		 */
		timer_clear_idle();
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		/*
		 * We've not stopped the tick yet, and there's a timer in the
		 * next period, so no point in stopping it either, bail.
		 */
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		if (!ts->tick_stopped) {
			tick = 0;
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			goto out;
		}
	}

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	/*
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	 * 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
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	 * don't drop this here the jiffies might be stale and
	 * do_timer() never invoked. Keep track of the fact that it
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	 * was the one which had the do_timer() duty last. If this CPU
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	 * is the one which had the do_timer() duty last, we limit the
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	 * sleep time to the timekeeping max_deferment value.
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	 * Otherwise we can sleep as long as we want.
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	 */
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	delta = timekeeping_max_deferment();
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	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) {
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		delta = KTIME_MAX;
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		ts->do_timer_last = 0;
	} else if (!ts->do_timer_last) {
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		delta = KTIME_MAX;
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	}
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#ifdef CONFIG_NO_HZ_FULL
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	/* Limit the tick delta to the maximum scheduler deferment */
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	if (!ts->inidle)
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		delta = min(delta, scheduler_tick_max_deferment());
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#endif

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	/* Calculate the next expiry time */
	if (delta < (KTIME_MAX - basemono))
		expires = basemono + delta;
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	else
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		expires = KTIME_MAX;

	expires = min_t(u64, expires, next_tick);
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	tick = expires;
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	/* Skip reprogram of event if its not changed */
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	if (ts->tick_stopped && (expires == ts->next_tick)) {
		/* Sanity check: make sure clockevent is actually programmed */
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		if (tick == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer))
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			goto out;
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		WARN_ON_ONCE(1);
		printk_once("basemono: %llu ts->next_tick: %llu dev->next_event: %llu timer->active: %d timer->expires: %llu\n",
			    basemono, ts->next_tick, dev->next_event,
			    hrtimer_active(&ts->sched_timer), hrtimer_get_expires(&ts->sched_timer));
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	}
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	/*
	 * 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) {
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		calc_load_nohz_start();
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		cpu_load_update_nohz_start();
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		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
		ts->tick_stopped = 1;
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		trace_tick_stop(1, TICK_DEP_MASK_NONE);
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	}
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	ts->next_tick = tick;

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

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	if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
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		hrtimer_start_expires(&ts->sched_timer, HRTIMER_MODE_ABS_PINNED);
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	else
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		tick_program_event(tick, 1);
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out:
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	/*
	 * Update the estimated sleep length until the next timer
	 * (not only the tick).
	 */
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	ts->sleep_length = ktime_sub(dev->next_event, now);
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	return tick;
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}

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static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
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{
	/* Update jiffies first */
	tick_do_update_jiffies64(now);
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	cpu_load_update_nohz_stop();
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	/*
	 * Clear the timer idle flag, so we avoid IPIs on remote queueing and
	 * the clock forward checks in the enqueue path:
	 */
	timer_clear_idle();

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

	tick_nohz_restart(ts, now);
}
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static void tick_nohz_full_update_tick(struct tick_sched *ts)
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{
#ifdef CONFIG_NO_HZ_FULL
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	int cpu = smp_processor_id();
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	if (!tick_nohz_full_cpu(cpu))
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		return;
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	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
		return;
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	if (can_stop_full_tick(cpu, ts))
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		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
	else if (ts->tick_stopped)
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		tick_nohz_restart_sched_tick(ts, ktime_get());
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#endif
}

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static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
{
	/*
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	 * If this CPU is offline and it is the one which updates
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	 * jiffies, then give up the assignment and let it be taken by
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	 * the CPU which runs the tick timer next. If we don't drop
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	 * 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;
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		/*
		 * Make sure the CPU doesn't get fooled by obsolete tick
		 * deadline if it comes back online later.
		 */
		ts->next_tick = 0;
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		return false;
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	}

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

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

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

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	if (tick_nohz_full_enabled()) {
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		/*
		 * 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;
	}

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	return true;
}

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

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

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