[PATCH] tick/nohz: Update comments some more

From: Ingo Molnar
Date: Thu Sep 28 2023 - 05:07:13 EST


* tip-bot2 for Frederic Weisbecker <tip-bot2@xxxxxxxxxxxxx> wrote:

> Author: Frederic Weisbecker <frederic@xxxxxxxxxx>
> AuthorDate: Tue, 12 Sep 2023 12:44:03 +02:00
> Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CommitterDate: Wed, 27 Sep 2023 16:58:10 +02:00
>
> tick/nohz: Update obsolete comments

Speling nits:

> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c

> /**
> + * 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-eavaluate the next tick and try to keep it stopped

s/re-eavaluate
/re-evaluate

> + * as long as possible.
> + * 2.2) If the tick has dependencies, restart the tick.
> *
> */
> void tick_nohz_idle_exit(void)
> {
> @@ -1364,7 +1384,13 @@ void tick_nohz_idle_exit(void)
> }
>
> /*
> + * In low-resolution mode, the tick handler must be implemented directly
> + * at the clockevent level. hrtimer can't be used instead because its

s/instead because
/instead, because

> + * 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.

commodity?

> */
> static void tick_nohz_lowres_handler(struct clock_event_device *dev)
> {

As well-deserved penace for my nitpicking, I've fixed these on top of
tip:timers/core, and have also done a full scan of kernel/time/tick-sched.c
for spelling, consistency of style and readability of comments - see
the patch below.

Thanks,

Ingo

===========================>
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Thu, 28 Sep 2023 10:45:54 +0200
Subject: [PATCH] tick/nohz: Update comments some more

Inspired by recent enhancements to comments in kernel/time/tick-sched.c,
go through the entire file and fix/unify its comments:

- Fix over a dozen typos, spelling mistakes & cases of bad grammar.

- Re-phrase sentences that I needed to read three times to understand.

[ I used the following arbitrary rule-of-thumb:
- if I had to read a comment twice, it was usually my fault,
- if I had to read it a third time, it was the comment's fault. ]

- Comma updates:

- Add commas where needed

- Remove commas where not needed

- In cases where a comma is optional, choose one variant and try to
standardize it over similar sentences in the file.

- Standardize on standalone 'NOHZ' spelling in free-flowing comments:

s/nohz/NOHZ
s/no idle tick/NOHZ

Still keep 'dynticks' as a popular synonym.

- Standardize on referring to variable names within free-flowing
comments with the "'var'" nomenclature, and function names as
"function_name()".

- Standardize on '64-bit' and '32-bit':
s/32bit/32-bit
s/64bit/64-bit

- Standardize on 'IRQ work':
s/irq work/IRQ work

- A few other tidyups I probably missed to list.

No change in functionality intended - other than one small change to
a syslog output string.

Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/time/tick-sched.c | 148 +++++++++++++++++++++++------------------------
1 file changed, 73 insertions(+), 75 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 8e9a9dcf60d5..60f1fe653f52 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)
@@ -1219,7 +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
@@ -1263,7 +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,
@@ -1307,9 +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;
/*
@@ -1351,7 +1351,7 @@ static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
*
* 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.
*
@@ -1385,7 +1385,7 @@ void tick_nohz_idle_exit(void)

/*
* 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()).
*
@@ -1426,7 +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)
{
@@ -1440,8 +1440,8 @@ 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 */
@@ -1464,7 +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
@@ -1483,7 +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)
{
@@ -1509,8 +1509,8 @@ static enum hrtimer_restart tick_nohz_highres_handler(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);
@@ -1548,16 +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_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());
@@ -1607,10 +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.
*/