tick/nohz: Update comments some more

 *  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

 *  NOHZ implementation for low and high resolution timers

 *

 *  Started by: Thomas Gleixner and Ingo Molnar

 */

#if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS)

/*

 * The time, when the last jiffy update happened. Write access must hold

 * The time when the last jiffy update happened. Write access must hold

 * jiffies_lock and jiffies_seq. tick_nohz_next_event() needs to get a

 * consistent view of jiffies and last_jiffies_update.

 */

	ktime_t delta, nextp;

	/*

	 * 64bit can do a quick check without holding jiffies lock and

	 * 64-bit can do a quick check without holding the jiffies lock and

	 * without looking at the sequence count. The smp_load_acquire()

	 * pairs with the update done later in this function.

	 *

	 * 32bit cannot do that because the store of tick_next_period

	 * consists of two 32bit stores and the first store could move it

	 * to a random point in the future.

	 * 32-bit cannot do that because the store of 'tick_next_period'

	 * consists of two 32-bit stores, and the first store could be

	 * moved by the CPU to a random point in the future.

	 */

	if (IS_ENABLED(CONFIG_64BIT)) {

		if (ktime_before(now, smp_load_acquire(&tick_next_period)))

		unsigned int seq;

		/*

		 * Avoid contention on jiffies_lock and protect the quick

		 * Avoid contention on 'jiffies_lock' and protect the quick

		 * check with the sequence count.

		 */

		do {

	/* Quick check failed, i.e. update is required. */

	raw_spin_lock(&jiffies_lock);

	/*

	 * Reevaluate with the lock held. Another CPU might have done the

	 * Re-evaluate with the lock held. Another CPU might have done the

	 * update already.

	 */

	if (ktime_before(now, tick_next_period)) {

						   TICK_NSEC);

	}

	/* Advance jiffies to complete the jiffies_seq protected job */

	/* Advance jiffies to complete the 'jiffies_seq' protected job */

	jiffies_64 += ticks;

	/*

	 * Keep the tick_next_period variable up to date.

	 */

	/* Keep the tick_next_period variable up to date */

	nextp = ktime_add_ns(last_jiffies_update, TICK_NSEC);

	if (IS_ENABLED(CONFIG_64BIT)) {

		/*

		 * Pairs with smp_load_acquire() in the lockless quick

		 * check above and ensures that the update to jiffies_64 is

		 * not reordered vs. the store to tick_next_period, neither

		 * check above, and ensures that the update to 'jiffies_64' is

		 * not reordered vs. the store to 'tick_next_period', neither

		 * by the compiler nor by the CPU.

		 */

		smp_store_release(&tick_next_period, nextp);

	} else {

		/*

		 * A plain store is good enough on 32bit as the quick check

		 * A plain store is good enough on 32-bit, as the quick check

		 * above is protected by the sequence count.

		 */

		tick_next_period = nextp;

	/*

	 * Release the sequence count. calc_global_load() below is not

	 * protected by it, but jiffies_lock needs to be held to prevent

	 * protected by it, but 'jiffies_lock' needs to be held to prevent

	 * concurrent invocations.

	 */

	write_seqcount_end(&jiffies_seq);

	raw_spin_lock(&jiffies_lock);

	write_seqcount_begin(&jiffies_seq);

	/* Did we start the jiffies update yet ? */

	/* Have we started the jiffies update yet ? */

	if (last_jiffies_update == 0) {

		u32 rem;

		last_jiffies_update = tick_next_period;

	}

	period = last_jiffies_update;

	write_seqcount_end(&jiffies_seq);

	raw_spin_unlock(&jiffies_lock);

	return period;

}

	 * concurrency: This happens only when the CPU in charge went

	 * into a long sleep. If two CPUs happen to assign themselves to

	 * this duty, then the jiffies update is still serialized by

	 * jiffies_lock.

	 * 'jiffies_lock'.

	 *

	 * If nohz_full is enabled, this should not happen because the

	 * tick_do_timer_cpu never relinquishes.

	 * 'tick_do_timer_cpu' CPU never relinquishes.

	 */

	if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {

#ifdef CONFIG_NO_HZ_FULL

	}

#endif

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

	/* Check if jiffies need an update */

	if (tick_do_timer_cpu == cpu)

		tick_do_update_jiffies64(now);

	/*

	 * If jiffies update stalled for too long (timekeeper in stop_machine()

	 * If the jiffies update stalled for too long (timekeeper in stop_machine()

	 * or VMEXIT'ed for several msecs), force an update.

	 */

	if (ts->last_tick_jiffies != jiffies) {

	/*

	 * 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

	 * time. This happens on completely 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.

	 * when we go busy again does not account too many ticks.

	 */

	if (ts->tick_stopped) {

		touch_softlockup_watchdog_sched();

	/*

	 * If the task is not running, run_posix_cpu_timers()

	 * has nothing to elapse, IPI can then be spared.

	 * has nothing to elapse, and an IPI can then be optimized out.

	 *

	 * activate_task()                      STORE p->tick_dep_mask

	 *   STORE p->on_rq

/*

 * Set a global tick dependency. Used by perf events that rely on freq and

 * by unstable clock.

 * unstable clocks.

 */

void tick_nohz_dep_set(enum tick_dep_bits bit)

{

/*

 * Set per-CPU tick dependency. Used by scheduler and perf events in order to

 * manage events throttling.

 * manage event-throttling.

 */

void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)

{

		if (cpu == smp_processor_id()) {

			tick_nohz_full_kick();

		} else {

			/* Remote irq work not NMI-safe */

			/* Remote IRQ work not NMI-safe */

			if (!WARN_ON_ONCE(in_nmi()))

				tick_nohz_full_kick_cpu(cpu);

		}

EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);

/*

 * Set a per-task tick dependency. RCU need this. Also posix CPU timers

 * Set a per-task tick dependency. RCU needs this. Also posix CPU timers

 * in order to elapse per task timers.

 */

void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)

bool tick_nohz_cpu_hotpluggable(unsigned int cpu)

{

	/*

	 * The tick_do_timer_cpu CPU handles housekeeping duty (unbound

	 * The 'tick_do_timer_cpu' CPU handles housekeeping duty (unbound

	 * timers, workqueues, timekeeping, ...) on behalf of full dynticks

	 * CPUs. It must remain online when nohz full is enabled.

	 */

		return;

	/*

	 * 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

	 * 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()) {

		pr_warn("NO_HZ: Can't run full dynticks because arch doesn't support irq work self-IPIs\n");

		pr_warn("NO_HZ: Can't run full dynticks because arch doesn't support IRQ work self-IPIs\n");

		cpumask_clear(tick_nohz_full_mask);

		tick_nohz_full_running = false;

		return;

 * 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.

 * CPU, which has the update task assigned, is in a long sleep.

 */

static void tick_nohz_update_jiffies(ktime_t now)

{

 * counters if NULL.

 *

 * Return the cumulative idle time (since boot) for a given

 * CPU, in microseconds. Note this is partially broken due to

 * CPU, in microseconds. Note that this is partially broken due to

 * the counter of iowait tasks that can be remotely updated without

 * any synchronization. Therefore it is possible to observe backward

 * values within two consecutive reads.

	}

	/*

	 * Reset to make sure next tick stop doesn't get fooled by past

	 * Reset to make sure the next tick stop doesn't get fooled by past

	 * cached clock deadline.

	 */

	ts->next_tick = 0;

	/*

	 * Keep the periodic tick, when RCU, architecture or irq_work

	 * requests it.

	 * Aside of that check whether the local timer softirq is

	 * pending. If so its a bad idea to call get_next_timer_interrupt()

	 * Aside of that, check whether the local timer softirq is

	 * pending. If so, its a bad idea to call get_next_timer_interrupt(),

	 * because there is an already expired timer, so it will request

	 * immediate expiry, which rearms the hardware timer with a

	 * minimal delta which brings us back to this place

	 * minimal delta, which brings us back to this place

	 * immediately. Lather, rinse and repeat...

	 */

	if (rcu_needs_cpu() || arch_needs_cpu() ||

	/*

	 * If this CPU is the one which had the do_timer() duty last, we limit

	 * the sleep time to the timekeeping max_deferment value.

	 * the sleep time to the timekeeping 'max_deferment' value.

	 * Otherwise we can sleep as long as we want.

	 */

	delta = timekeeping_max_deferment();

	 * If this CPU is the one which updates jiffies, then give up

	 * the assignment and let it be taken by the CPU which runs

	 * the tick timer next, which might be this CPU as well. If we

	 * don't drop this here the jiffies might be stale and

	 * do_timer() never invoked. Keep track of the fact that it

	 * don't drop this here, the jiffies might be stale and

	 * do_timer() never gets invoked. Keep track of the fact that it

	 * was the one which had the do_timer() duty last.

	 */

	if (cpu == tick_do_timer_cpu) {

		ts->do_timer_last = 0;

	}

	/* Skip reprogram of event if its not changed */

	/* Skip reprogram of event if it's not changed */

	if (ts->tick_stopped && (expires == ts->next_tick)) {

		/* Sanity check: make sure clockevent is actually programmed */

		if (tick == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer))

	}

	/*

	 * nohz_stop_sched_tick can be called several times before

	 * the nohz_restart_sched_tick is called. This happens when

	 * nohz_stop_sched_tick() can be called several times before

	 * 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.

	 * the scheduler tick in nohz_restart_sched_tick().

	 */

	if (!ts->tick_stopped) {

		calc_load_nohz_start();

	calc_load_nohz_stop();

	touch_softlockup_watchdog_sched();

	/*

	 * Cancel the scheduled timer and restore the tick

	 */

	/* Cancel the scheduled timer and restore the tick: */

	ts->tick_stopped  = 0;

	tick_nohz_restart(ts, now);

}

/*

 * A pending softirq outside an IRQ (or softirq disabled section) context

 * should be waiting for ksoftirqd to handle it. Therefore we shouldn't

 * reach here due to the need_resched() early check in can_stop_idle_tick().

 * reach this code due to the need_resched() early check in can_stop_idle_tick().

 *

 * However if we are between CPUHP_AP_SMPBOOT_THREADS and CPU_TEARDOWN_CPU on the

 * cpu_down() process, softirqs can still be raised while ksoftirqd is parked,

 * triggering the below since wakep_softirqd() is ignored.

 * triggering the code below, since wakep_softirqd() is ignored.

 *

 */

static bool report_idle_softirq(void)

	if (ratelimit >= 10)

		return false;

	/* On RT, softirqs handling may be waiting on some lock */

	/* On RT, softirq handling may be waiting on some lock */

	if (local_bh_blocked())

		return false;

	 * 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.

	 * this here, the jiffies might be stale and do_timer() never

	 * gets invoked.

	 */

	if (unlikely(!cpu_online(cpu))) {

		if (cpu == tick_do_timer_cpu)

/**

 * tick_nohz_get_next_hrtimer - return the next expiration time for the hrtimer

 * or the tick, whatever that expires first. Note that, if the tick has been

 * or the tick, whichever expires first. Note that, if the tick has been

 * stopped, it returns the next hrtimer.

 *

 * Called from power state control code with interrupts disabled

		return *delta_next;

	/*

	 * If the next highres timer to expire is earlier than next_event, the

	 * If the next highres timer to expire is earlier than 'next_event', the

	 * idle governor needs to know that.

	 */

	next_event = min_t(u64, next_event,

	if (vtime_accounting_enabled_this_cpu())

		return;

	/*

	 * We stopped the tick in idle. Update process times would miss the

	 * time we slept as update_process_times does only a 1 tick

	 * accounting. Enforce that this is accounted to idle !

	 * We stopped the tick in idle. update_process_times() would miss the

	 * time we slept, as it does only a 1 tick accounting.

	 * Enforce that this is accounted to idle !

	 */

	ticks = jiffies - ts->idle_jiffies;

	/*

 *

 * 2) If the CPU is in nohz_full mode (corner case):

 *   2.1) If the tick can be kept stopped (no tick dependencies)

 *        then re-eavaluate the next tick and try to keep it stopped

 *        then re-evaluate the next tick and try to keep it stopped

 *        as long as possible.

 *   2.2) If the tick has dependencies, restart the tick.

 *

/*

 * In low-resolution mode, the tick handler must be implemented directly

 * at the clockevent level. hrtimer can't be used instead because its

 * at the clockevent level. hrtimer can't be used instead, because its

 * infrastructure actually relies on the tick itself as a backend in

 * low-resolution mode (see hrtimer_run_queues()).

 *

 * This low-resolution handler still makes use of some hrtimer APIs meanwhile

 * for commodity with expiration calculation and forwarding.

 * for convenience with expiration calculation and forwarding.

 */

static void tick_nohz_lowres_handler(struct clock_event_device *dev)

{

}

/**

 * tick_nohz_switch_to_nohz - switch to nohz mode

 * tick_nohz_switch_to_nohz - switch to NOHZ mode

 */

static void tick_nohz_switch_to_nohz(void)

{

		return;

	/*

	 * Recycle the hrtimer in ts, so we can share the

	 * hrtimer_forward with the highres code.

	 * Recycle the hrtimer in 'ts', so we can share the

	 * hrtimer_forward_now() function with the highres code.

	 */

	hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);

	/* Get the next period */

	if (ts->idle_active)

		tick_nohz_stop_idle(ts, now);

	/*

	 * If all CPUs are idle. We may need to update a stale jiffies value.

	 * If all CPUs are idle we may need to update a stale jiffies value.

	 * Note nohz_full is a special case: a timekeeper is guaranteed to stay

	 * alive but it might be busy looping with interrupts disabled in some

	 * rare case (typically stop machine). So we must make sure we have a

#endif /* CONFIG_NO_HZ_COMMON */

/*

 * Called from irq_enter to notify about the possible interruption of idle()

 * Called from irq_enter() to notify about the possible interruption of idle()

 */

void tick_irq_enter(void)

{

	tick_sched_do_timer(ts, now);

	/*

	 * Do not call, when we are not in irq context and have

	 * no valid regs pointer

	 * Do not call when we are not in IRQ context and have

	 * no valid 'regs' pointer

	 */

	if (regs)

		tick_sched_handle(ts, regs);

	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	ktime_t now = ktime_get();

	/*

	 * Emulate tick processing via per-CPU hrtimers:

	 */

	/* Emulate tick processing via per-CPU hrtimers: */

	hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);

	ts->sched_timer.function = tick_nohz_highres_handler;

	/* Get the next period (per-CPU) */

	hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());

	/* Offset the tick to avert jiffies_lock contention. */

	/* Offset the tick to avert 'jiffies_lock' contention. */

	if (sched_skew_tick) {

		u64 offset = TICK_NSEC >> 1;

		do_div(offset, num_possible_cpus());

}

/*

 * Check, if a change happened, which makes oneshot possible.

 * 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

 * Called cyclically 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). Called with interrupts disabled.

 */