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procfs: do not overflow get_{idle,iowait}_time for nohz

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Since commit a25cac5198 ("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 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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michal Hocko 2011-12-08 14:34:32 -08:00 committed by Linus Torvalds
parent 1368edf064
commit 2a95ea6c0d
1 changed files with 2 additions and 2 deletions
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4
fs/proc/stat.c
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@ -32,7 +32,7 @@ static cputime64_t get_idle_time(int cpu)
idle = kstat_cpu(cpu).cpustat.idle; idle = kstat_cpu(cpu).cpustat.idle;
idle = cputime64_add(idle, arch_idle_time(cpu)); idle = cputime64_add(idle, arch_idle_time(cpu));
} else } else
idle = usecs_to_cputime(idle_time); idle = nsecs_to_jiffies64(1000 * idle_time);
return idle; return idle;
} }
@ -46,7 +46,7 @@ static cputime64_t get_iowait_time(int cpu)
/* !NO_HZ so we can rely on cpustat.iowait */ /* !NO_HZ so we can rely on cpustat.iowait */
iowait = kstat_cpu(cpu).cpustat.iowait; iowait = kstat_cpu(cpu).cpustat.iowait;
else else
iowait = usecs_to_cputime(iowait_time); iowait = nsecs_to_jiffies64(1000 * iowait_time);
return iowait; return iowait;
} }