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Subject[tip:timers/core] hrtimer: Make hrtimer_cpu_base.next_timer handling unconditional
Commit-ID:  eb27926ba05233dc4f2052cc9d4f19359ec3cd2c
Author: Anna-Maria Gleixner <>
AuthorDate: Thu, 21 Dec 2017 11:41:45 +0100
Committer: Ingo Molnar <>
CommitDate: Tue, 16 Jan 2018 02:35:47 +0100

hrtimer: Make hrtimer_cpu_base.next_timer handling unconditional

hrtimer_cpu_base.next_timer stores the pointer to the next expiring timer
in a CPU base.

This pointer cannot be dereferenced and is solely used to check whether a
hrtimer which is removed is the hrtimer which is the first to expire in the
CPU base. If this is the case, then the timer hardware needs to be
reprogrammed to avoid an extra interrupt for nothing.

Again, this is conditional functionality, but there is no compelling reason
to make this conditional. As a preparation, hrtimer_cpu_base.next_timer
needs to be available unconditonally.

Aside of that the upcoming support for softirq based hrtimers requires access
to this pointer unconditionally as well, so our motivation is not entirely
simplicity based.

Make the update of hrtimer_cpu_base.next_timer unconditional and remove the
#ifdef cruft. The impact on CONFIG_HIGH_RES_TIMERS=n && CONFIG_NOHZ=n is
marginal as it's just a store on an already dirtied cacheline.

No functional change.

Signed-off-by: Anna-Maria Gleixner <>
Cc: Christoph Hellwig <>
Cc: John Stultz <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
include/linux/hrtimer.h | 4 ++--
kernel/time/hrtimer.c | 12 ++----------
2 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index bb7270e..2d3e1d6 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -164,13 +164,13 @@ enum hrtimer_base_type {
* @hres_active: State of high resolution mode
* @in_hrtirq: hrtimer_interrupt() is currently executing
* @hang_detected: The last hrtimer interrupt detected a hang
- * @next_timer: Pointer to the first expiring timer
* @nr_events: Total number of hrtimer interrupt events
* @nr_retries: Total number of hrtimer interrupt retries
* @nr_hangs: Total number of hrtimer interrupt hangs
* @max_hang_time: Maximum time spent in hrtimer_interrupt
* @expires_next: absolute time of the next event, is required for remote
* hrtimer enqueue
+ * @next_timer: Pointer to the first expiring timer
* @clock_base: array of clock bases for this cpu
* Note: next_timer is just an optimization for __remove_hrtimer().
@@ -186,13 +186,13 @@ struct hrtimer_cpu_base {
unsigned int in_hrtirq : 1,
hang_detected : 1;
- struct hrtimer *next_timer;
unsigned int nr_events;
unsigned short nr_retries;
unsigned short nr_hangs;
unsigned int max_hang_time;
ktime_t expires_next;
+ struct hrtimer *next_timer;
struct hrtimer_clock_base clock_base[HRTIMER_MAX_CLOCK_BASES];
} ____cacheline_aligned;

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index a9ab67f..26abaa7 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -459,21 +459,13 @@ __next_base(struct hrtimer_cpu_base *cpu_base, unsigned int *active)
while ((base = __next_base((cpu_base), &(active))))

-static inline void hrtimer_update_next_timer(struct hrtimer_cpu_base *cpu_base,
- struct hrtimer *timer)
- cpu_base->next_timer = timer;
static ktime_t __hrtimer_get_next_event(struct hrtimer_cpu_base *cpu_base)
struct hrtimer_clock_base *base;
unsigned int active = cpu_base->active_bases;
ktime_t expires, expires_next = KTIME_MAX;

- hrtimer_update_next_timer(cpu_base, NULL);
+ cpu_base->next_timer = NULL;
for_each_active_base(base, cpu_base, active) {
struct timerqueue_node *next;
struct hrtimer *timer;
@@ -483,7 +475,7 @@ static ktime_t __hrtimer_get_next_event(struct hrtimer_cpu_base *cpu_base)
expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
if (expires < expires_next) {
expires_next = expires;
- hrtimer_update_next_timer(cpu_base, timer);
+ cpu_base->next_timer = timer;
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