procfs: do not overflow get_{idle,iowait}_time for nohz
Since
a25cac51 ("proc: Consider NO_HZ when printing idle and iowait
times") we are reporting idle/io_wait time also while a CPU is tickless.
We rely on get_{idle,iowait}_time functions to retrieve proper data.
These functions, however, 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 parameter overflows jiffies_to_usecs(MAX_JIFFY_OFFSET) and returns
MAX_JIFFY_OFFSET in that case.
When do we overflow depends on CONFIG_HZ but especially for CONFIG_HZ_300
it is quite low (
1431649781) so we are getting MAX_JIFFY_OFFSET for
>3000s! until we overflow unsigned int. Just for reference CONFIG_HZ_100
has an overflow window around 20s, CONFIG_HZ_250 ~8s and CONFIG_HZ_1000
~2s.
This results in a bug when people saw [h]top going mad reporting 100% CPU
usage even though there was basically no CPU load. The reason was simply
that /proc/stat stopped reporting idle/io_wait changes (and reported
MAX_JIFFY_OFFSET) and so the only change happening was for user system
time.
Let's use nsecs_to_jiffies64 instead which doesn't reduce the precision to
32b type and it is much more appropriate for cumulative time values
(unlike usecs_to_jiffies which intended for timeout calculations).
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Tested-by: Artem S. Tashkinov <t.artem@mailcity.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>