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    SubjectRe: Re: [PATCH] proc: Do not overflow get_{idle,iowait}_time for nohz (was: Re: Re: [REGRESSION] [Linux 3.2] top/htop and all other CPU usage)
    On Fri 02-12-11 20:12:14, Artem S. Tashkinov wrote:
    > On Dec 2, 2011, Michal Hocko wrote:
    >
    > > And the one with a more cleaned up changelog. No functional changes
    > > ---
    > > From 107887016b91de59194a93c751d040b05d5e37fe Mon Sep 17 00:00:00 2001
    > > From: Michal Hocko <>
    > > 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_100 has an overflow window around 20s,
    > > CONFIG_250 ~8s and CONFIG_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>
    > > ---
    > > 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
    >
    > Thank you, this patch has fixed the issue for me.
    >
    > Tested-by: Artem S. Tashkinov <t.artem@mailcity.com>

    Thanks for retesting!

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


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