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Message-Id: <20251129133526.1460119-3-jackzxcui1989@163.com>
Date: Sat, 29 Nov 2025 21:35:26 +0800
From: Xin Zhao <jackzxcui1989@....com>
To: anna-maria@...utronix.de,
frederic@...nel.org,
mingo@...nel.org,
tglx@...utronix.de,
kuba@...nel.org
Cc: jackzxcui1989@....com,
linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: [PATCH 2/2] timers/nohz: Avoid /proc/stat idle/iowait fluctuation when cpu hotplug
The idle and iowait statistics in /proc/stat are obtained through
get_idle_time and get_iowait_time. Assuming CONFIG_NO_HZ_COMMON is
enabled, when CPU is online, the idle and iowait values use the
idle_sleeptime and iowait_sleeptime statistics from tick_cpu_sched, but
use CPUTIME_IDLE and CPUTIME_IOWAIT items from kernel_cpustat when CPU
is offline. Although /proc/stat do not print statistics of offline CPU,
it still print aggregated statistics for all possible CPUs.
tick_cpu_sched and kernel_cpustat are maintained by different logic,
leading to a significant gap. The first line of the data below shows the
/proc/stat output when only one CPU remains after CPU offline, the second
line shows the /proc/stat output after all CPUs are brought back online:
cpu 2408558 2 916619 4275883 5403 123758 64685 0 0 0
cpu 2408588 2 916693 4200737 4184 123762 64686 0 0 0
Obviously, other values do not experience significant fluctuations, while
idle/iowait statistics show a substantial decrease, which make system CPU
monitoring troublesome.
Introduce get_cpu_idle_time_us_raw and get_cpu_iowait_time_us_raw, so that
/proc/stat logic can use them to get the last raw value of idle_sleeptime
and iowait_sleeptime from tick_cpu_sched without any calculation when CPU
is offline. It avoids /proc/stat idle/iowait fluctuation when cpu hotplug.
Signed-off-by: Xin Zhao <jackzxcui1989@....com>
---
fs/proc/stat.c | 4 ++++
include/linux/tick.h | 4 ++++
kernel/time/tick-sched.c | 46 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 54 insertions(+)
diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 8b444e862..de13a2e1c 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -28,6 +28,8 @@ u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
if (cpu_online(cpu))
idle_usecs = get_cpu_idle_time_us(cpu, NULL);
+ else
+ idle_usecs = get_cpu_idle_time_us_raw(cpu);
if (idle_usecs == -1ULL)
/* !NO_HZ or cpu offline so we can rely on cpustat.idle */
@@ -44,6 +46,8 @@ static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)
if (cpu_online(cpu))
iowait_usecs = get_cpu_iowait_time_us(cpu, NULL);
+ else
+ iowait_usecs = get_cpu_iowait_time_us_raw(cpu);
if (iowait_usecs == -1ULL)
/* !NO_HZ or cpu offline so we can rely on cpustat.iowait */
diff --git a/include/linux/tick.h b/include/linux/tick.h
index ac76ae9fa..bdd38d270 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -139,7 +139,9 @@ extern ktime_t tick_nohz_get_next_hrtimer(void);
extern ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next);
extern unsigned long tick_nohz_get_idle_calls_cpu(int cpu);
extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
+extern u64 get_cpu_idle_time_us_raw(int cpu);
extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
+extern u64 get_cpu_iowait_time_us_raw(int cpu);
#else /* !CONFIG_NO_HZ_COMMON */
#define tick_nohz_enabled (0)
static inline int tick_nohz_tick_stopped(void) { return 0; }
@@ -161,7 +163,9 @@ static inline ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
return *delta_next;
}
static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
+static inline u64 get_cpu_idle_time_us_raw(int cpu) { return -1; }
static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
+static inline u64 get_cpu_iowait_time_us_raw(int cpu) { return -1; }
#endif /* !CONFIG_NO_HZ_COMMON */
/*
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 4d089b290..607fc22d4 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -829,6 +829,29 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
}
EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
+/**
+ * get_cpu_idle_time_us_raw - get the raw total idle time of a CPU
+ * @cpu: CPU number to query
+ *
+ * Return the raw idle time (since boot) for a given CPU, in
+ * microseconds.
+ *
+ * This time is measured via accounting rather than sampling,
+ * and is as accurate as ktime_get() is.
+ *
+ * This function returns -1 if NOHZ is not enabled.
+ */
+u64 get_cpu_idle_time_us_raw(int cpu)
+{
+ struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+
+ if (!tick_nohz_active)
+ return -1;
+
+ return ktime_to_us(ts->idle_sleeptime);
+}
+EXPORT_SYMBOL_GPL(get_cpu_idle_time_us_raw);
+
/**
* get_cpu_iowait_time_us - get the total iowait time of a CPU
* @cpu: CPU number to query
@@ -855,6 +878,29 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
}
EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
+/**
+ * get_cpu_iowait_time_us_raw - get the raw total iowait time of a CPU
+ * @cpu: CPU number to query
+ *
+ * Return the raw iowait time (since boot) for a given CPU, in
+ * microseconds.
+ *
+ * This time is measured via accounting rather than sampling,
+ * and is as accurate as ktime_get() is.
+ *
+ * This function returns -1 if NOHZ is not enabled.
+ */
+u64 get_cpu_iowait_time_us_raw(int cpu)
+{
+ struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+
+ if (!tick_nohz_active)
+ return -1;
+
+ return ktime_to_us(ts->iowait_sleeptime);
+}
+EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us_raw);
+
static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
hrtimer_cancel(&ts->sched_timer);
--
2.34.1
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