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fs/proc/uptime.c: Fix idle time reporting in /proc/uptime

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/proc/uptime reports idle time by reading the CPUTIME_IDLE field from
the per-cpu kcpustats. However, on NO_HZ systems, idle time is not
continually updated on idle cpus, leading this value to appear
incorrectly small.

/proc/stat performs an accounting update when reading idle time; we
can use the same approach for uptime.

With this patch, /proc/stat and /proc/uptime now agree on idle time.
Additionally, the following shows idle time tick up consistently on an
idle machine:

  (while true; do cat /proc/uptime; sleep 1; done) | awk '{print $2-prev; prev=$2}'

Reported-by: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lkml.kernel.org/r/20210827165438.3280779-1-joshdon@google.com
This commit is contained in:
Josh Don 2021-08-27 09:54:38 -07:00 committed by Peter Zijlstra
parent bc9ffef31b
commit a130e8fbc7
3 changed files with 12 additions and 7 deletions
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4
fs/proc/stat.c
View File

@ -24,7 +24,7 @@
#ifdef arch_idle_time
static u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
{
u64 idle;
@ -46,7 +46,7 @@ static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)
#else
static u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
{
u64 idle, idle_usecs = -1ULL;

14
fs/proc/uptime.c
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@ -12,18 +12,22 @@ static int uptime_proc_show(struct seq_file *m, void *v)
{
struct timespec64 uptime;
struct timespec64 idle;
u64 nsec;
u64 idle_nsec;
u32 rem;
int i;
nsec = 0;
for_each_possible_cpu(i)
nsec += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE];
idle_nsec = 0;
for_each_possible_cpu(i) {
struct kernel_cpustat kcs;
kcpustat_cpu_fetch(&kcs, i);
idle_nsec += get_idle_time(&kcs, i);
}
ktime_get_boottime_ts64(&uptime);
timens_add_boottime(&uptime);
idle.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem);
idle.tv_sec = div_u64_rem(idle_nsec, NSEC_PER_SEC, &rem);
idle.tv_nsec = rem;
seq_printf(m, "%lu.%02lu %lu.%02lu\n",
(unsigned long) uptime.tv_sec,

1
include/linux/kernel_stat.h
View File

@ -102,6 +102,7 @@ extern void account_system_index_time(struct task_struct *, u64,
enum cpu_usage_stat);
extern void account_steal_time(u64);
extern void account_idle_time(u64);
extern u64 get_idle_time(struct kernel_cpustat *kcs, int cpu);
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
static inline void account_process_tick(struct task_struct *tsk, int user)