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Message-Id: <20251210083135.3993562-3-jackzxcui1989@163.com>
Date: Wed, 10 Dec 2025 16:31:35 +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 v2 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 of 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.
get_cpu_idle_time_us() calculates the latest cpu idle time based on
idle_entrytime and current time. When CPU is idle when offline, the value
return by get_cpu_idle_time_us() will continue to increase, which is
unexpected. get_cpu_iowait_time_us() has the similar calculation logic.
When CPU is in the iowait state when offline, the value return by
get_cpu_iowait_time_us() will continue to increase.
Introduce get_cpu_idle_time_us_offline() as the _offline variants of
get_cpu_idle_time_us(). get_cpu_idle_time_us_offline() just return the
same value of idle_sleeptime without any calculation. In this way,
/proc/stat logic can use it to get a correct CPU idle time, which remains
unchanged during CPU offline period. Also, the aggregated statistics of
all possible CPUs printed by /proc/stat will not experience significant
fluctuation when CPU hotplug.
So as the newly added get_cpu_iowait_time_us_offline().
Signed-off-by: Xin Zhao <jackzxcui1989@....com>
---
fs/proc/stat.c | 4 +++
include/linux/tick.h | 4 +++
kernel/time/tick-sched.c | 54 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 62 insertions(+)
diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 8b444e862..9920e7bfc 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_offline(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_offline(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..e5db27657 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_offline(int cpu);
extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
+extern u64 get_cpu_iowait_time_us_offline(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_offline(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_offline(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..2cda08f94 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -829,6 +829,33 @@ 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_offline - get the total idle time of an offline CPU
+ * @cpu: CPU number to query
+ *
+ * Return the idle time (since boot) for a given offline CPU, in microseconds.
+ *
+ * This function does not calculate the latest idle time based on
+ * idle_entrytime and current time like get_cpu_idle_time_us() does.
+ * If get_cpu_idle_time_us() is used to obtain idle time of an offline CPU,
+ * its value will continue to increase, which is unexpected.
+ *
+ * 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_offline(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_offline);
+
/**
* get_cpu_iowait_time_us - get the total iowait time of a CPU
* @cpu: CPU number to query
@@ -855,6 +882,33 @@ 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_offline - get the total iowait time of an offline CPU
+ * @cpu: CPU number to query
+ *
+ * Return the iowait time (since boot) for a given CPU, in microseconds.
+ *
+ * This function does not calculate the latest iowait time based on
+ * idle_entrytime and current time like get_cpu_iowait_time_us() does.
+ * If get_cpu_iowait_time_us is used to obtain iowait time of an offline CPU,
+ * its value will continue to increase, which is unexpected.
+ *
+ * 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_offline(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_offline);
+
static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
hrtimer_cancel(&ts->sched_timer);
--
2.34.1
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