[tip: sched/core] fs/proc/uptime.c: Fix idle time reporting in /proc/uptime

[tip-bot2 for Josh Don]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     1f74f9ea5a0cc9147f5bda4b5715ccb5cc34605d
Gitweb:        https://git.kernel.org/tip/1f74f9ea5a0cc9147f5bda4b5715ccb5cc34605d
Author:        Josh Don <joshdon@xxxxxxxxxx>
AuthorDate:    Fri, 27 Aug 2021 09:54:38 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 09 Sep 2021 11:27:31 +02:00

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

/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@xxxxxxxxxx>
Signed-off-by: Josh Don <joshdon@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210827165438.3280779-1-joshdon@xxxxxxxxxx
---
 fs/proc/stat.c              |  4 ++--
 fs/proc/uptime.c            | 14 +++++++++-----
 include/linux/kernel_stat.h |  1 +
 3 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 6561a06..4fb8729 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -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;
 
diff --git a/fs/proc/uptime.c b/fs/proc/uptime.c
index 5a1b228..deb99bc 100644
--- a/fs/proc/uptime.c
+++ b/fs/proc/uptime.c
@@ -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,
diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 44ae1a7..69ae6b2 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -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)