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

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Message-ID: <f271abd3-e1b6-0968-abf8-6f1b74e21f65@loongson.cn>
Date: Tue, 12 Aug 2025 18:48:48 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Xianglai Li <lixianglai@...ngson.cn>, Huacai Chen
 <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
 Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>,
 Song Gao <gaosong@...ngson.cn>, Tianrui Zhao <zhaotianrui@...ngson.cn>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: time: Fix the issue of high cpu usage of vcpu
 threads in virtual machines



On 2025/8/12 下午5:00, Xianglai Li 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
During timer is turned off, timer interrupt may be arriving. Here adding 
EOI with timer after disabling it so that there is no pending timer 
interrupt when offline cpu is halted state.
> 
> Signed-off-by: Xianglai Li <lixianglai@...ngson.cn>
> ---
>   arch/loongarch/kernel/time.c | 20 ++++++++++++++++++++
>   include/linux/cpuhotplug.h   |  1 +
>   2 files changed, 21 insertions(+)
> 
> diff --git a/arch/loongarch/kernel/time.c b/arch/loongarch/kernel/time.c
> index 367906b10f81..4daa11512eba 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>
> @@ -86,6 +87,9 @@ static int constant_set_state_shutdown(struct clock_event_device *evt)
>   	timer_config &= ~CSR_TCFG_EN;
>   	csr_write64(timer_config, LOONGARCH_CSR_TCFG);
>   
> +	/* Clear Timer Interrupt */
> +	write_csr_tintclear(CSR_TINTCLR_TI);
> +
>   	raw_spin_unlock(&state_lock);
>   
>   	return 0;
> @@ -208,8 +212,17 @@ int __init constant_clocksource_init(void)
>   	return res;
>   }
>   
> +static int arch_timer_dying_cpu(unsigned int cpu)
> +{
NULL is a little strange here, how about something like this?

         struct clock_event_device *evt;

         evt = this_cpu_ptr(constant_clockevent_device);
         constant_set_state_shutdown(evt);

Regards
Bibo Mao

> +	constant_set_state_shutdown(NULL);
> +
> +	return 0;
> +}
> +
>   void __init time_init(void)
>   {
> +	int err;
> +
>   	if (!cpu_has_cpucfg)
>   		const_clock_freq = cpu_clock_freq;
>   	else
> @@ -220,4 +233,11 @@ void __init time_init(void)
>   	constant_clockevent_init();
>   	constant_clocksource_init();
>   	pv_time_init();
> +
> +	err = cpuhp_setup_state_nocalls(CPUHP_AP_LOONGARCH_ARCH_TIMER_STARTING,
> +					"loongarch/timer:starting",
> +					NULL, arch_timer_dying_cpu);
> +	if (err)
> +		pr_info("cpu hotplug event register failed");
> +
>   }
> 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,
> 
> base-commit: 53e760d8949895390e256e723e7ee46618310361
>