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-H7uhvZeZ9L40AWuRN7t4JAFLNDj4YUOZ_K-oPrCcnpEjA@mail.gmail.com>
Date: Tue, 12 Aug 2025 21:07:53 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: lixianglai <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] LoongArch: time: Fix the issue of high cpu usage of vcpu
 threads in virtual machines

On Tue, Aug 12, 2025 at 8:50 PM lixianglai <lixianglai@...ngson.cn> wrote:
>
>
>
> Hi Huacai Chen:
> > Hi, Xianglai,
> >
> > There is something that can be improved.
> >
> > On Tue, Aug 12, 2025 at 5:24 PM 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
> >>
> >> 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)
> > We can use arch_timer_dying() for short. And then add an
> > arch_timer_starting() like this:
> >
> > static int arch_timer_starting(unsigned int cpu)
> > {
> >          set_csr_ecfg(ECFGF_TIMER);
> >
> >          return 0;
> > }
> >
> > Though ECFGF_TIMER may be enabled in other places, for syntax we need it here.
> >
> >> +{
> >> +       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);
> > Then we need use cpuhp_setup_state() here, because we have a startup
> > function now.
> >
> > And "loongarch/timer:starting" should be
> > "clockevents/loongarch/timer:starting" like others.
> >
> > And the whole should be moved to the last of
> > constant_clockevent_init() because it is clockevent specific.
>
> like this?
>
> @@ -164,6 +182,10 @@ int constant_clockevent_init(void)
>
>          timer_irq_installed = 1;
>
> +       cpuhp_setup_state(CPUHP_AP_LOONGARCH_ARCH_TIMER_STARTING,
> +                         "clockevents/loongarch/timer:starting",
> +                         arch_timer_starting, arch_timer_dying_cpu);
> +
>          sync_counter();
>
>
> I was wondering whether it should be placed before or after the
> "timer_irq_installed" judgment
Should be after "timer_irq_installed" because we only need to run once.

The best place is after pr_info("Constant clock event device register\n");

And there is another question:
Should we move "write_csr_tintclear(CSR_TINTCLR_TI)" from
constant_set_state_shutdown() to arch_timer_dying()?


Huacai

>
>
> >> +       if (err)
> >> +               pr_info("cpu hotplug event register failed");
> > This is not so useful, because the error isn't fatal.
> >
> >
> > Huacai
> >
> >> +
> >>   }
> >> 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
> >> --
> >> 2.39.1
> >>
>
>