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

Commit-ID:     0abec32a6836eca6b61ae81e4829f94abd4647c7
Gitweb:        https://git.kernel.org/tip/0abec32a6836eca6b61ae81e4829f94abd4647c7
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Tue, 24 Feb 2026 17:36:06 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 27 Feb 2026 16:40:06 +01:00

sched/hrtick: Mark hrtick timer LAZY_REARM

The hrtick timer is frequently rearmed before expiry and most of the time
the new expiry is past the armed one. As this happens on every context
switch it becomes expensive with scheduling heavy work loads especially in
virtual machines as the "hardware" reprogamming implies a VM exit.

hrtimer now provide a lazy rearm mode flag which skips the reprogamming if:

    1) The timer was the first expiring timer before the rearm

    2) The new expiry time is farther out than the armed time

This avoids a massive amount of reprogramming operations of the hrtick
timer for the price of eventually taking the alredy armed interrupt for
nothing.

Mark the hrtick timer accordingly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163429.475409346@kernel.org
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5bc446e..2d1239a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -998,7 +998,8 @@ static void hrtick_rq_init(struct rq *rq)
 {
 	INIT_CSD(&rq->hrtick_csd, __hrtick_start, rq);
 	rq->hrtick_sched = HRTICK_SCHED_NONE;
-	hrtimer_setup(&rq->hrtick_timer, hrtick, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
+	hrtimer_setup(&rq->hrtick_timer, hrtick, CLOCK_MONOTONIC,
+		      HRTIMER_MODE_REL_HARD | HRTIMER_MODE_LAZY_REARM);
 }
 #else /* !CONFIG_SCHED_HRTICK: */
 static inline void hrtick_clear(struct rq *rq) { }