[PATCH] proc: Do not overflow get_{idle,iowait}_time for nohz

[Michal Hocko]

get_{idle,iowait}_time use usecs_to_cputime to translate micro seconds
time to cputime64_t. This is just an alias to usecs_to_jiffies which
reduces the data type from u64 to unsigned int and also checks whether
the given paramerer overflows jiffies_to_usecs(MAX_JIFFY_OFFSET) and
returns MAX_JIFFY_OFFSET in that case. How much we overflow depends on
CONFIG_HZ and especially for CONFIG_HZ_300 it is quite low (1431649781).

This results in a bug when people saw [h]top going mad reporting 100%
CPU usage even though there was basically no CPU load at all. The reason
was simply that /proc/stat stopped reporting idle/io_wait changes (and
reported MAX_JIFFY_OFFSET) and so the only change happenning was for
user system time.

Let's use nsecs_to_jiffies64 instead as it doesn't overflow.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 fs/proc/stat.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 42b274d..2a30d67 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -32,7 +32,7 @@ static cputime64_t get_idle_time(int cpu)
 		idle = kstat_cpu(cpu).cpustat.idle;
 		idle = cputime64_add(idle, arch_idle_time(cpu));
 	} else
-		idle = usecs_to_cputime(idle_time);
+		idle = nsecs_to_jiffies64(1000 * idle_time);
 
 	return idle;
 }
@@ -46,7 +46,7 @@ static cputime64_t get_iowait_time(int cpu)
 		/* !NO_HZ so we can rely on cpustat.iowait */
 		iowait = kstat_cpu(cpu).cpustat.iowait;
 	else
-		iowait = usecs_to_cputime(iowait_time);
+		iowait = nsecs_to_jiffies64(1000 * iowait_time);
 
 	return iowait;
 }
-- 
1.7.7.3


-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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