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

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Message-Id: <20250812090056.771379-1-lixianglai@loongson.cn>
Date: Tue, 12 Aug 2025 17:00:56 +0800
From: Xianglai Li <lixianglai@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
	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>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH] LoongArch: time: Fix the issue of high cpu usage of vcpu threads in virtual machines

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)
+{
+	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
-- 
2.39.1