[patch 39/48] hrtimer: Avoid re-evaluation when nothing changed
From: Thomas Gleixner
Date: Tue Feb 24 2026 - 11:51:46 EST
Most times there is no change between hrtimer_interrupt() deferring the rearm
and the invocation of hrtimer_rearm_deferred(). In those cases it's a pointless
exercise to re-evaluate the next expiring timer.
Cache the required data and use it if nothing changed.
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx
---
include/linux/hrtimer_defs.h | 53 +++++++++++++++++++++----------------------
kernel/time/hrtimer.c | 45 +++++++++++++++++++++++++-----------
2 files changed, 58 insertions(+), 40 deletions(-)
--- a/include/linux/hrtimer_defs.h
+++ b/include/linux/hrtimer_defs.h
@@ -47,32 +47,31 @@ enum hrtimer_base_type {
/**
* struct hrtimer_cpu_base - the per cpu clock bases
- * @lock: lock protecting the base and associated clock bases
- * and timers
- * @cpu: cpu number
- * @active_bases: Bitfield to mark bases with active timers
- * @clock_was_set_seq: Sequence counter of clock was set events
- * @hres_active: State of high resolution mode
- * @deferred_rearm: A deferred rearm is pending
- * @hang_detected: The last hrtimer interrupt detected a hang
- * @softirq_activated: displays, if the softirq is raised - update of softirq
- * related settings is not required then.
- * @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
- * @softirq_expiry_lock: Lock which is taken while softirq based hrtimer are
- * expired
- * @online: CPU is online from an hrtimers point of view
- * @timer_waiters: A hrtimer_cancel() invocation waits for the timer
- * callback to finish.
- * @expires_next: absolute time of the next event, is required for remote
- * hrtimer enqueue; it is the total first expiry time (hard
- * and soft hrtimer are taken into account)
- * @next_timer: Pointer to the first expiring timer
- * @softirq_expires_next: Time to check, if soft queues needs also to be expired
- * @softirq_next_timer: Pointer to the first expiring softirq based timer
- * @clock_base: array of clock bases for this cpu
+ * @lock: lock protecting the base and associated clock bases and timers
+ * @cpu: cpu number
+ * @active_bases: Bitfield to mark bases with active timers
+ * @clock_was_set_seq: Sequence counter of clock was set events
+ * @hres_active: State of high resolution mode
+ * @deferred_rearm: A deferred rearm is pending
+ * @deferred_needs_update: The deferred rearm must re-evaluate the first timer
+ * @hang_detected: The last hrtimer interrupt detected a hang
+ * @softirq_activated: displays, if the softirq is raised - update of softirq
+ * related settings is not required then.
+ * @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
+ * @softirq_expiry_lock: Lock which is taken while softirq based hrtimer are expired
+ * @online: CPU is online from an hrtimers point of view
+ * @timer_waiters: A hrtimer_cancel() waiters for the timer callback to finish.
+ * @expires_next: Absolute time of the next event, is required for remote
+ * hrtimer enqueue; it is the total first expiry time (hard
+ * and soft hrtimer are taken into account)
+ * @next_timer: Pointer to the first expiring timer
+ * @softirq_expires_next: Time to check, if soft queues needs also to be expired
+ * @softirq_next_timer: Pointer to the first expiring softirq based timer
+ * @deferred_expires_next: Cached expires next value for deferred rearm
+ * @clock_base: Array of clock bases for this cpu
*
* Note: next_timer is just an optimization for __remove_hrtimer().
* Do not dereference the pointer because it is not reliable on
@@ -85,6 +84,7 @@ struct hrtimer_cpu_base {
unsigned int clock_was_set_seq;
bool hres_active;
bool deferred_rearm;
+ bool deferred_needs_update;
bool hang_detected;
bool softirq_activated;
bool online;
@@ -102,6 +102,7 @@ struct hrtimer_cpu_base {
struct hrtimer *next_timer;
ktime_t softirq_expires_next;
struct hrtimer *softirq_next_timer;
+ ktime_t deferred_expires_next;
struct hrtimer_clock_base clock_base[HRTIMER_MAX_CLOCK_BASES];
call_single_data_t csd;
} ____cacheline_aligned;
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -919,8 +919,10 @@ static bool update_needs_ipi(struct hrti
return false;
/* If a deferred rearm is pending the remote CPU will take care of it */
- if (cpu_base->deferred_rearm)
+ if (cpu_base->deferred_rearm) {
+ cpu_base->deferred_needs_update = true;
return false;
+ }
/*
* Walk the affected clock bases and check whether the first expiring
@@ -1141,7 +1143,12 @@ static void __remove_hrtimer(struct hrti
* a local timer is removed to be immediately restarted. That's handled
* at the call site.
*/
- if (reprogram && timer == cpu_base->next_timer && !timer->is_lazy)
+ if (!reprogram || timer != cpu_base->next_timer || timer->is_lazy)
+ return;
+
+ if (cpu_base->deferred_rearm)
+ cpu_base->deferred_needs_update = true;
+ else
hrtimer_force_reprogram(cpu_base, /* skip_equal */ true);
}
@@ -1328,8 +1335,10 @@ static bool __hrtimer_start_range_ns(str
}
/* If a deferred rearm is pending skip reprogramming the device */
- if (cpu_base->deferred_rearm)
+ if (cpu_base->deferred_rearm) {
+ cpu_base->deferred_needs_update = true;
return false;
+ }
if (!was_first || cpu_base != this_cpu_base) {
/*
@@ -1939,8 +1948,7 @@ static __latent_entropy void hrtimer_run
* Very similar to hrtimer_force_reprogram(), except it deals with
* deferred_rearm and hang_detected.
*/
-static void hrtimer_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t now,
- ktime_t expires_next, bool deferred)
+static void hrtimer_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t expires_next, bool deferred)
{
cpu_base->expires_next = expires_next;
cpu_base->deferred_rearm = false;
@@ -1950,7 +1958,7 @@ static void hrtimer_rearm(struct hrtimer
* Give the system a chance to do something else than looping
* on hrtimer interrupts.
*/
- expires_next = ktime_add_ns(now, 100 * NSEC_PER_MSEC);
+ expires_next = ktime_add_ns(ktime_get(), 100 * NSEC_PER_MSEC);
cpu_base->hang_detected = false;
}
hrtimer_rearm_event(expires_next, deferred);
@@ -1960,27 +1968,36 @@ static void hrtimer_rearm(struct hrtimer
void __hrtimer_rearm_deferred(void)
{
struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
- ktime_t now, expires_next;
+ ktime_t expires_next;
if (!cpu_base->deferred_rearm)
return;
guard(raw_spinlock)(&cpu_base->lock);
- now = hrtimer_update_base(cpu_base);
- expires_next = hrtimer_update_next_event(cpu_base);
- hrtimer_rearm(cpu_base, now, expires_next, true);
+ if (cpu_base->deferred_needs_update) {
+ hrtimer_update_base(cpu_base);
+ expires_next = hrtimer_update_next_event(cpu_base);
+ } else {
+ /* No timer added/removed. Use the cached value */
+ expires_next = cpu_base->deferred_expires_next;
+ }
+ hrtimer_rearm(cpu_base, expires_next, true);
}
static __always_inline void
-hrtimer_interrupt_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t now, ktime_t expires_next)
+hrtimer_interrupt_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t expires_next)
{
+ /* hrtimer_interrupt() just re-evaluated the first expiring timer */
+ cpu_base->deferred_needs_update = false;
+ /* Cache the expiry time */
+ cpu_base->deferred_expires_next = expires_next;
set_thread_flag(TIF_HRTIMER_REARM);
}
#else /* CONFIG_HRTIMER_REARM_DEFERRED */
static __always_inline void
-hrtimer_interrupt_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t now, ktime_t expires_next)
+hrtimer_interrupt_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t expires_next)
{
- hrtimer_rearm(cpu_base, now, expires_next, false);
+ hrtimer_rearm(cpu_base, expires_next, false);
}
#endif /* !CONFIG_HRTIMER_REARM_DEFERRED */
@@ -2041,7 +2058,7 @@ void hrtimer_interrupt(struct clock_even
cpu_base->hang_detected = true;
}
- hrtimer_interrupt_rearm(cpu_base, now, expires_next);
+ hrtimer_interrupt_rearm(cpu_base, expires_next);
raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
}