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: <da4311b0-7e8e-647a-260f-1733878cf394@loongson.cn>
Date: Tue, 12 Aug 2025 20:48:56 +0800
From: lixianglai <lixianglai@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
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



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


>> +       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
>>