From b7dd64778aa3f89de9afa1e81171cfe110ddc525 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 24 Feb 2026 17:36:01 +0100 Subject: hrtimer: Provide LAZY_REARM mode 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. Add 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. Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260224163429.408524456@kernel.org --- include/linux/hrtimer.h | 8 ++++++++ include/linux/hrtimer_types.h | 3 +++ 2 files changed, 11 insertions(+) (limited to 'include/linux') diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h index b5003856fd60..c924bb2498db 100644 --- a/include/linux/hrtimer.h +++ b/include/linux/hrtimer.h @@ -31,6 +31,13 @@ * soft irq context * HRTIMER_MODE_HARD - Timer callback function will be executed in * hard irq context even on PREEMPT_RT. + * HRTIMER_MODE_LAZY_REARM - Avoid reprogramming if the timer was the + * first expiring timer and is moved into the + * future. Special mode for the HRTICK timer to + * avoid extensive reprogramming of the hardware, + * which is expensive in virtual machines. Risks + * a pointless expiry, but that's better than + * reprogramming on every context switch, */ enum hrtimer_mode { HRTIMER_MODE_ABS = 0x00, @@ -38,6 +45,7 @@ enum hrtimer_mode { HRTIMER_MODE_PINNED = 0x02, HRTIMER_MODE_SOFT = 0x04, HRTIMER_MODE_HARD = 0x08, + HRTIMER_MODE_LAZY_REARM = 0x10, HRTIMER_MODE_ABS_PINNED = HRTIMER_MODE_ABS | HRTIMER_MODE_PINNED, HRTIMER_MODE_REL_PINNED = HRTIMER_MODE_REL | HRTIMER_MODE_PINNED, diff --git a/include/linux/hrtimer_types.h b/include/linux/hrtimer_types.h index 8fbbb6bdf7a1..64381c64cdbd 100644 --- a/include/linux/hrtimer_types.h +++ b/include/linux/hrtimer_types.h @@ -33,6 +33,8 @@ enum hrtimer_restart { * @is_soft: Set if hrtimer will be expired in soft interrupt context. * @is_hard: Set if hrtimer will be expired in hard interrupt context * even on RT. + * @is_lazy: Set if the timer is frequently rearmed to avoid updates + * of the clock event device * * The hrtimer structure must be initialized by hrtimer_setup() */ @@ -45,6 +47,7 @@ struct hrtimer { u8 is_rel; u8 is_soft; u8 is_hard; + u8 is_lazy; }; #endif /* _LINUX_HRTIMER_TYPES_H */ -- cgit v1.2.3