Messages in this thread Patch in this message |  | | | Date | Mon, 5 Dec 2011 09:55:59 +0100 | | From | Michal Hocko <> | | Subject | [PATCH for 3.2] proc: Do not overflow get_{idle,iowait}_time for nohz |
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Hi Andrew, Thomas, this is a regression fix (reported https://lkml.org/lkml/2011/11/24/102) introduced by an earlier patch merged in 3.2-rc4. I am not sure what is the correct channel so I am adding both of you. The original nohz /proc/stat fixlet was picked by Andrew and later merged by Thomas IIRC.
Thanks!
--- From 1c74dec8565ef56f815dbbde333214e9e14410b8 Mon Sep 17 00:00:00 2001 From: Michal Hocko <mhocko@suse.cz> Date: Fri, 2 Dec 2011 16:17:03 +0100 Subject: [PATCH] proc: 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> --- 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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