[PATCH 13/15] sched/cputime: Provide get_cpu_[idle|iowait]_time_us() off-case

Frederic Weisbecker
2026-02-06T14:24:44Z

The last reason why get_cpu_idle/iowait_time_us() may return -1 now is
if the config doesn't support nohz.

The ad-hoc replacement solution by cpufreq is to compute jiffies minus
the whole busy cputime. Although the intention should provide a coherent
low resolution estimation of the idle and iowait time, the
implementation is buggy because jiffies don't start at 0.

Just provide instead a real get_cpu_[idle|iowait]_time_us() offcase.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
drivers/cpufreq/cpufreq.c | 29 +----------------------------
include/linux/kernel_stat.h | 3 +++
include/linux/tick.h | 4 ----
kernel/sched/cputime.c | 12 +++++++++---
4 files changed, 13 insertions(+), 35 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 4472bb1ec83c..ecb9634cd06b 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -130,38 +130,11 @@ struct kobject *get_governor_parent_kobj(struct cpufreq_policy *policy)
}
EXPORT_SYMBOL_GPL(get_governor_parent_kobj);

-static inline u64 get_cpu_idle_time_jiffy(unsigned int cpu, u64 *wall)
-{
- struct kernel_cpustat kcpustat;
- u64 cur_wall_time;
- u64 idle_time;
- u64 busy_time;
-
- cur_wall_time = jiffies64_to_nsecs(get_jiffies_64());
-
- kcpustat_cpu_fetch(&kcpustat, cpu);
-
- busy_time = kcpustat.cpustat[CPUTIME_USER];
- busy_time += kcpustat.cpustat[CPUTIME_SYSTEM];
- busy_time += kcpustat.cpustat[CPUTIME_IRQ];
- busy_time += kcpustat.cpustat[CPUTIME_SOFTIRQ];
- busy_time += kcpustat.cpustat[CPUTIME_STEAL];
- busy_time += kcpustat.cpustat[CPUTIME_NICE];
-
- idle_time = cur_wall_time - busy_time;
- if (wall)
- *wall = div_u64(cur_wall_time, NSEC_PER_USEC);
-
- return div_u64(idle_time, NSEC_PER_USEC);
-}
-
u64 get_cpu_idle_time(unsigned int cpu, u64 *wall, int io_busy)
{
u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);

- if (idle_time == -1ULL)
- return get_cpu_idle_time_jiffy(cpu, wall);
- else if (!io_busy)
+ if (!io_busy)
idle_time += get_cpu_iowait_time_us(cpu, wall);

return idle_time;
diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 3680519d7b2c..512104b0ff49 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -133,6 +133,9 @@ static inline bool kcpustat_idle_dyntick(void)
}
#endif /* CONFIG_NO_HZ_COMMON */

+extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
+extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
+
/* Fetch cputime values when vtime is disabled on a CPU */
static inline u64 kcpustat_field_default(enum cpu_usage_stat usage, int cpu)
{
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 738007d6f577..1cf4651f09ad 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -139,8 +139,6 @@ extern bool tick_nohz_idle_got_tick(void);
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_iowait_time_us(int cpu, u64 *last_update_time);
#else /* !CONFIG_NO_HZ_COMMON */
#define tick_nohz_enabled (0)
static inline bool tick_nohz_is_active(void) { return false; }
@@ -162,8 +160,6 @@ static inline ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
*delta_next = TICK_NSEC;
return *delta_next;
}
-static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
-static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
#endif /* !CONFIG_NO_HZ_COMMON */

/*
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 057fdc00dbc6..d588a4a50e57 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -509,6 +509,13 @@ u64 kcpustat_field_iowait(int cpu)
nr_iowait_cpu(cpu), ktime_get());
}
EXPORT_SYMBOL_GPL(kcpustat_field_iowait);
+#else
+static u64 kcpustat_field_dyntick(int cpu, enum cpu_usage_stat idx,
+ bool compute_delta, ktime_t now)
+{
+ return kcpustat_cpu(cpu).cpustat[idx];
+}
+#endif /* CONFIG_NO_HZ_COMMON */

static u64 get_cpu_sleep_time_us(int cpu, enum cpu_usage_stat idx,
bool compute_delta, u64 *last_update_time)
@@ -544,7 +551,7 @@ static u64 get_cpu_sleep_time_us(int cpu, enum cpu_usage_stat idx,
* This time is measured via accounting rather than sampling,
* and is as accurate as ktime_get() is.
*
- * Return: -1 if generic vtime is enabled, else total idle time of the @cpu
+ * Return: total idle time of the @cpu
*/
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
@@ -568,7 +575,7 @@ EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
* This time is measured via accounting rather than sampling,
* and is as accurate as ktime_get() is.
*
- * Return: -1 if generic vtime is enabled, else total iowait time of @cpu
+ * Return: total iowait time of @cpu
*/
u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
{
@@ -576,7 +583,6 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
nr_iowait_cpu(cpu), last_update_time);
}
EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
-#endif /* CONFIG_NO_HZ_COMMON */

/*
* Use precise platform statistics if available:
--
2.51.1