[tip: sched/hrtick] sched/hrtick: Avoid tiny hrtick rearms

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/hrtick branch of tip:

Commit-ID:     c8cdb9b516407a0b8c653c9c1d6f0931c3864384
Gitweb:        https://git.kernel.org/tip/c8cdb9b516407a0b8c653c9c1d6f0931c3864384
Author:        Thomas Gleixner <tglx@xxxxxxxxxx>
AuthorDate:    Tue, 24 Feb 2026 17:35:56 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 27 Feb 2026 16:40:05 +01:00

sched/hrtick: Avoid tiny hrtick rearms

Tiny adjustments to the hrtick expiry time below 5 microseconds are just
causing extra work for no real value. Filter them out when restarting the
hrtick.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260224163429.340593047@xxxxxxxxxx
---
 kernel/sched/core.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a868f0a..5bc446e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -903,12 +903,24 @@ static enum hrtimer_restart hrtick(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-static void __hrtick_restart(struct rq *rq)
+static inline bool hrtick_needs_rearm(struct hrtimer *timer, ktime_t expires)
+{
+	/*
+	 * Queued is false when the timer is not started or currently
+	 * running the callback. In both cases, restart. If queued check
+	 * whether the expiry time actually changes substantially.
+	 */
+	return !hrtimer_is_queued(timer) ||
+		abs(expires - hrtimer_get_expires(timer)) > 5000;
+}
+
+static void hrtick_cond_restart(struct rq *rq)
 {
 	struct hrtimer *timer = &rq->hrtick_timer;
 	ktime_t time = rq->hrtick_time;
 
-	hrtimer_start(timer, time, HRTIMER_MODE_ABS_PINNED_HARD);
+	if (hrtick_needs_rearm(timer, time))
+		hrtimer_start(timer, time, HRTIMER_MODE_ABS_PINNED_HARD);
 }
 
 /*
@@ -920,7 +932,7 @@ static void __hrtick_start(void *arg)
 	struct rq_flags rf;
 
 	rq_lock(rq, &rf);
-	__hrtick_restart(rq);
+	hrtick_cond_restart(rq);
 	rq_unlock(rq, &rf);
 }
 
@@ -950,9 +962,11 @@ void hrtick_start(struct rq *rq, u64 delay)
 	}
 
 	rq->hrtick_time = ktime_add_ns(ktime_get(), delta);
+	if (!hrtick_needs_rearm(&rq->hrtick_timer, rq->hrtick_time))
+		return;
 
 	if (rq == this_rq())
-		__hrtick_restart(rq);
+		hrtimer_start(&rq->hrtick_timer, rq->hrtick_time, HRTIMER_MODE_ABS_PINNED_HARD);
 	else
 		smp_call_function_single_async(cpu_of(rq), &rq->hrtick_csd);
 }
@@ -966,7 +980,7 @@ static inline void hrtick_schedule_exit(struct rq *rq)
 {
 	if (rq->hrtick_sched & HRTICK_SCHED_START) {
 		rq->hrtick_time = ktime_add_ns(ktime_get(), rq->hrtick_delay);
-		__hrtick_restart(rq);
+		hrtick_cond_restart(rq);
 	} else if (idle_rq(rq)) {
 		/*
 		 * No need for using hrtimer_is_active(). The timer is CPU local