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: <CAAhV-H72TWAwxV5bcHMej_8=cH1GxJVJxKMASWpPXVcq++FUWg@mail.gmail.com>
Date: Tue, 12 Aug 2025 19:21:28 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: Xianglai Li <lixianglai@...ngson.cn>, 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>, 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 Tue, Aug 12, 2025 at 6:50 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> 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);
Both OK for me, if really needed, we can use
constant_set_state_shutdown(this_cpu_ptr(constant_clockevent_device))
for short.


Huacai

>
> 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
> >
>
>