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

[lixianglai]

Hi Huacai Chen:
> Hi, Xianglai,
>
> There is something that can be improved.
>
> On Tue, Aug 12, 2025 at 5:24 PM Xianglai Li <lixianglai@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >   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


> > +       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