Merge tag 'timers-core-2023-10-29-v2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer updates from Thomas Gleixner:
 "Updates for time, timekeeping and timers:

  Core:

   - Avoid superfluous deactivation of the tick in the low resolution
     tick NOHZ interrupt handler as the deactivation is handled already
     in the idle loop and on interrupt exit.

   - Update stale comments in the tick NOHZ code and rename the tick
     handler functions to be self-explanatory.

   - Remove an unused function in the tick NOHZ code, which was
     forgotten when the last user went away.

   - Handle RTC alarms which exceed the maximum alarm time of the
     underlying RTC hardware gracefully.

     Setting RTC alarms which exceed the maximum alarm time of the RTC
     hardware failed so far and caused suspend operations to abort.

     Cure this by limiting the alarm to the maximum alarm time of the
     RTC hardware, which is provided by the driver. This causes early
     resume wakeups, but that's way better than not suspending at all.

  Drivers:

   - Add a proper clocksource/event driver for the ancient Cirrus Logic
     EP93xx SoC family, which is one of the last non device-tree
     holdouts in arch/arm.

   - The usual boring device tree bindings updates and small fixes and
     enhancements all over the place"

* tag 'timers-core-2023-10-29-v2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  clocksource: ep93xx: Add driver for Cirrus Logic EP93xx
  dt-bindings: timers: Add Cirrus EP93xx
  clocksource/drivers/timer-atmel-tcb: Fix initialization on SAM9 hardware
  clocksource/timer-riscv: ACPI: Add timer_cannot_wakeup_cpu
  clocksource/drivers/sun5i: Remove surplus dev_err() when using platform_get_irq()
  drivers/clocksource/timer-ti-dm: Don't call clk_get_rate() in stop function
  clocksource/drivers/timer-imx-gpt: Fix potential memory leak
  dt-bindings: timer: renesas,rz-mtu3: Document RZ/{G2UL,Five} SoCs
  dt-bindings: timer: renesas,rz-mtu3: Improve documentation
  dt-bindings: timer: renesas,rz-mtu3: Fix overflow/underflow interrupt names
  alarmtimer: Use maximum alarm time for suspend
  rtc: Add API function to return alarm time bound by hardware limit
  tick/nohz: Update comments some more
  tick/nohz: Remove unused tick_nohz_idle_stop_tick_protected()
  tick/nohz: Don't shutdown the lowres tick from itself
  tick/nohz: Update obsolete comments
  tick/nohz: Rename the tick handlers to more self-explanatory names

2 files changed, 135 insertions, 98 deletions

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index 8d9f13d847f0..4657cb8e8b1f 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -290,6 +290,17 @@ static int alarmtimer_suspend(struct device *dev)
 	rtc_timer_cancel(rtc, &rtctimer);
 	rtc_read_time(rtc, &tm);
 	now = rtc_tm_to_ktime(tm);
+
+	/*
+	 * If the RTC alarm timer only supports a limited time offset, set the
+	 * alarm time to the maximum supported value.
+	 * The system may wake up earlier (possibly much earlier) than expected
+	 * when the alarmtimer runs. This is the best the kernel can do if
+	 * the alarmtimer exceeds the time that the rtc device can be programmed
+	 * for.
+	 */
+	min = rtc_bound_alarmtime(rtc, min);
+
 	now = ktime_add(now, min);
 
 	/* Set alarm, if in the past reject suspend briefly to handle */
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 87015e9deacc..be77b021e5d6 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -4,7 +4,7 @@
  *  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
  */
@@ -45,7 +45,7 @@ struct tick_sched *tick_get_tick_sched(int cpu)
 
 #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.
  */
@@ -60,13 +60,13 @@ static void tick_do_update_jiffies64(ktime_t now)
 	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)))
@@ -75,7 +75,7 @@ static void tick_do_update_jiffies64(ktime_t now)
 		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 {
@@ -90,7 +90,7 @@ static void tick_do_update_jiffies64(ktime_t now)
 	/* 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)) {
@@ -114,25 +114,23 @@ static void tick_do_update_jiffies64(ktime_t now)
 						   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;
@@ -140,7 +138,7 @@ static void tick_do_update_jiffies64(ktime_t now)
 
 	/*
 	 * 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);
@@ -160,7 +158,8 @@ static ktime_t tick_init_jiffy_update(void)
 
 	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;
 
@@ -175,8 +174,10 @@ static ktime_t tick_init_jiffy_update(void)
 		last_jiffies_update = tick_next_period;
 	}
 	period = last_jiffies_update;
+
 	write_seqcount_end(&jiffies_seq);
 	raw_spin_unlock(&jiffies_lock);
+
 	return period;
 }
 
@@ -192,10 +193,10 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	 * 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
@@ -205,12 +206,12 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	}
 #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) {
@@ -234,10 +235,10 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *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
+	 * 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();
@@ -362,7 +363,7 @@ static void tick_nohz_kick_task(struct task_struct *tsk)
 
 	/*
 	 * 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
@@ -425,7 +426,7 @@ static void tick_nohz_dep_set_all(atomic_t *dep,
 
 /*
  * 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)
 {
@@ -439,7 +440,7 @@ void tick_nohz_dep_clear(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)
 {
@@ -455,7 +456,7 @@ 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);
 		}
@@ -473,7 +474,7 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
 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)
@@ -546,7 +547,7 @@ void __init tick_nohz_full_setup(cpumask_var_t cpumask)
 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.
 	 */
@@ -568,12 +569,12 @@ void __init tick_nohz_init(void)
 		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;
@@ -643,7 +644,7 @@ bool tick_nohz_tick_stopped_cpu(int cpu)
  * 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)
 {
@@ -726,7 +727,7 @@ static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime,
  * 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.
@@ -787,7 +788,7 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 	}
 
 	/*
-	 * 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;
@@ -816,11 +817,11 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 	/*
 	 * 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() ||
@@ -861,7 +862,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int 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();
@@ -895,8 +896,8 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 	 * 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) {
@@ -906,7 +907,7 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int 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))
@@ -919,11 +920,11 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 	}
 
 	/*
-	 * 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();
@@ -985,9 +986,8 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
 
 	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);
 }
@@ -1019,11 +1019,11 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 /*
  * 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)
@@ -1044,7 +1044,7 @@ 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;
 
@@ -1061,8 +1061,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *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.
+	 * this here, the jiffies might be stale and do_timer() never
+	 * gets invoked.
 	 */
 	if (unlikely(!cpu_online(cpu))) {
 		if (cpu == tick_do_timer_cpu)
@@ -1175,12 +1175,23 @@ void tick_nohz_idle_enter(void)
 }
 
 /**
- * tick_nohz_irq_exit - update next tick event from interrupt exit
+ * tick_nohz_irq_exit - Notify the tick about IRQ exit
+ *
+ * A timer may have been added/modified/deleted either by the current IRQ,
+ * or by another place using this IRQ as a notification. This IRQ may have
+ * also updated the RCU callback list. These events may require a
+ * re-evaluation of the next tick. Depending on the context:
+ *
+ * 1) If the CPU is idle and no resched is pending, just proceed with idle
+ *    time accounting. The next tick will be re-evaluated on the next idle
+ *    loop iteration.
+ *
+ * 2) If the CPU is nohz_full:
  *
- * When an interrupt fires while we are idle and it doesn't cause
- * a reschedule, it may still add, modify or delete a timer, enqueue
- * an RCU callback, etc...
- * So we need to re-calculate and reprogram the next tick event.
+ *    2.1) If there is any tick dependency, restart the tick if stopped.
+ *
+ *    2.2) If there is no tick dependency, (re-)evaluate the next tick and
+ *         stop/update it accordingly.
  */
 void tick_nohz_irq_exit(void)
 {
@@ -1208,7 +1219,7 @@ bool tick_nohz_idle_got_tick(void)
 
 /**
  * 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
@@ -1252,7 +1263,7 @@ ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
 		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,
@@ -1296,9 +1307,9 @@ static void tick_nohz_account_idle_time(struct tick_sched *ts,
 	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;
 	/*
@@ -1330,11 +1341,20 @@ static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
 }
 
 /**
- * tick_nohz_idle_exit - restart the idle tick from the idle task
+ * tick_nohz_idle_exit - Update the tick upon idle task exit
+ *
+ * When the idle task exits, update the tick depending on the
+ * following situations:
+ *
+ * 1) If the CPU is not in nohz_full mode (most cases), then
+ *    restart the tick.
+ *
+ * 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-evaluate the next tick and try to keep it stopped
+ *        as long as possible.
+ *   2.2) If the tick has dependencies, restart the tick.
  *
- * Restart the idle tick when the CPU is woken up from idle
- * This also exit the RCU extended quiescent state. The CPU
- * can use RCU again after this function is called.
  */
 void tick_nohz_idle_exit(void)
 {
@@ -1364,9 +1384,15 @@ void tick_nohz_idle_exit(void)
 }
 
 /*
- * The nohz low res interrupt handler
+ * In low-resolution mode, the tick handler must be implemented directly
+ * 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 convenience with expiration calculation and forwarding.
  */
-static void tick_nohz_handler(struct clock_event_device *dev)
+static void tick_nohz_lowres_handler(struct clock_event_device *dev)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 	struct pt_regs *regs = get_irq_regs();
@@ -1377,18 +1403,16 @@ static void tick_nohz_handler(struct clock_event_device *dev)
 	tick_sched_do_timer(ts, now);
 	tick_sched_handle(ts, regs);
 
-	if (unlikely(ts->tick_stopped)) {
-		/*
-		 * The clockevent device is not reprogrammed, so change the
-		 * clock event device to ONESHOT_STOPPED to avoid spurious
-		 * interrupts on devices which might not be truly one shot.
-		 */
-		tick_program_event(KTIME_MAX, 1);
-		return;
+	/*
+	 * In dynticks mode, tick reprogram is deferred:
+	 * - to the idle task if in dynticks-idle
+	 * - to IRQ exit if in full-dynticks.
+	 */
+	if (likely(!ts->tick_stopped)) {
+		hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
+		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 	}
 
-	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
-	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 }
 
 static inline void tick_nohz_activate(struct tick_sched *ts, int mode)
@@ -1402,7 +1426,7 @@ static inline void tick_nohz_activate(struct tick_sched *ts, int mode)
 }
 
 /**
- * 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)
 {
@@ -1412,12 +1436,12 @@ static void tick_nohz_switch_to_nohz(void)
 	if (!tick_nohz_enabled)
 		return;
 
-	if (tick_switch_to_oneshot(tick_nohz_handler))
+	if (tick_switch_to_oneshot(tick_nohz_lowres_handler))
 		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 */
@@ -1440,7 +1464,7 @@ static inline void tick_nohz_irq_enter(void)
 	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
@@ -1459,7 +1483,7 @@ static inline void tick_nohz_activate(struct tick_sched *ts, int mode) { }
 #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)
 {
@@ -1475,7 +1499,7 @@ void tick_irq_enter(void)
  * We rearm the timer until we get disabled by the idle code.
  * Called with interrupts disabled.
  */
-static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
+static enum hrtimer_restart tick_nohz_highres_handler(struct hrtimer *timer)
 {
 	struct tick_sched *ts =
 		container_of(timer, struct tick_sched, sched_timer);
@@ -1485,15 +1509,19 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 	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);
 	else
 		ts->next_tick = 0;
 
-	/* No need to reprogram if we are in idle or full dynticks mode */
+	/*
+	 * In dynticks mode, tick reprogram is deferred:
+	 * - to the idle task if in dynticks-idle
+	 * - to IRQ exit if in full-dynticks.
+	 */
 	if (unlikely(ts->tick_stopped))
 		return HRTIMER_NORESTART;
 
@@ -1520,16 +1548,14 @@ void tick_setup_sched_timer(void)
 	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_sched_timer;
+	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());
@@ -1579,10 +1605,10 @@ void tick_oneshot_notify(void)
 }
 
 /*
- * 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.
  */