linux-kernel - Re: [PATCH V2] LoongArch: time: Fix the issue of high cpu usage of vcpu threads in virtual machines

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Message-ID: <CAAhV-H4+HQ9tZTKUgBqfkjDQnvhqVRcXy+yH0ZrYmCZoiv0BQA@mail.gmail.com>
Date: Thu, 21 Aug 2025 17:36:21 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xianglai Li <lixianglai@...ngson.cn>
Cc: WANG Xuerui <kernel@...0n.name>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Bibo Mao <maobibo@...ngson.cn>, 
	Song Gao <gaosong@...ngson.cn>, Tianrui Zhao <zhaotianrui@...ngson.cn>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] LoongArch: time: Fix the issue of high cpu usage of
 vcpu threads in virtual machines

Applied, thanks.

Huacai

On Wed, Aug 20, 2025 at 10:39 AM Xianglai Li <lixianglai@...ngson.cn> wrote:
>
> When the cpu is offline, the timer under loongarch is not correctly closed,
> resulting in an excessively high cpu usage rate of the offline vcpu thread
> in the virtual machine.
>
> To correctly close the timer, we have made the following modifications:
>
> Register the cpu hotplug timer start event for loongarch.This event will
> be called to close the timer when the cpu is offline.
>
> Clear the timer interrupt when the timer is turned off, Because before the
> timer is turned off, there may be a timer interrupt that has been in the
> pending state due to the interruption of the disabled, which also affects
> the halt state of the offline vcpu.
>
> Signed-off-by: Xianglai Li <lixianglai@...ngson.cn>
> ---
> V1->V2:
> 1.Add the implementation of the function arch_timer_starting
> 2.Add more descriptions about clear timer interrupts in the commit message
> 3.Delete useless prints
> 4.Adjust the calling position of the function cpuhp_setup_state
> 5.Adjust the timer interrupt clear position
>
>  arch/loongarch/kernel/time.c | 22 ++++++++++++++++++++++
>  include/linux/cpuhotplug.h   |  1 +
>  2 files changed, 23 insertions(+)
>
> diff --git a/arch/loongarch/kernel/time.c b/arch/loongarch/kernel/time.c
> index 367906b10f81..09b95f28bdcc 100644
> --- a/arch/loongarch/kernel/time.c
> +++ b/arch/loongarch/kernel/time.c
> @@ -12,6 +12,7 @@
>  #include <linux/kernel.h>
>  #include <linux/sched_clock.h>
>  #include <linux/spinlock.h>
> +#include <linux/cpu.h>
>
>  #include <asm/cpu-features.h>
>  #include <asm/loongarch.h>
> @@ -102,6 +103,23 @@ static int constant_timer_next_event(unsigned long delta, struct clock_event_dev
>         return 0;
>  }
>
> +static int arch_timer_dying_cpu(unsigned int cpu)
> +{
> +       constant_set_state_shutdown(this_cpu_ptr(&constant_clockevent_device));
> +
> +       /* Clear Timer Interrupt */
> +       write_csr_tintclear(CSR_TINTCLR_TI);
> +
> +       return 0;
> +}
> +
> +static int arch_timer_starting(unsigned int cpu)
> +{
> +       set_csr_ecfg(ECFGF_TIMER);
> +
> +       return 0;
> +}
> +
>  static unsigned long get_loops_per_jiffy(void)
>  {
>         unsigned long lpj = (unsigned long)const_clock_freq;
> @@ -172,6 +190,10 @@ int constant_clockevent_init(void)
>         lpj_fine = get_loops_per_jiffy();
>         pr_info("Constant clock event device register\n");
>
> +       cpuhp_setup_state(CPUHP_AP_LOONGARCH_ARCH_TIMER_STARTING,
> +                         "clockevents/loongarch/timer:starting",
> +                         arch_timer_starting, arch_timer_dying_cpu);
> +
>         return 0;
>  }
>
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index edfa61d80702..6606c1546afc 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -159,6 +159,7 @@ enum cpuhp_state {
>         CPUHP_AP_PERF_ARM_STARTING,
>         CPUHP_AP_PERF_RISCV_STARTING,
>         CPUHP_AP_ARM_L2X0_STARTING,
> +       CPUHP_AP_LOONGARCH_ARCH_TIMER_STARTING,
>         CPUHP_AP_EXYNOS4_MCT_TIMER_STARTING,
>         CPUHP_AP_ARM_ARCH_TIMER_STARTING,
>         CPUHP_AP_ARM_ARCH_TIMER_EVTSTRM_STARTING,
> --
> 2.39.1
>
>