Messages in this thread | | | Date | Fri, 16 Aug 2013 17:01:24 +0200 | From | Frederic Weisbecker <> | Subject | Re: proc/stat: idle goes backward |
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On Fri, Aug 16, 2013 at 04:47:04PM +0200, Oleg Nesterov wrote: > Hello. > > Out customer reports that "idle" field is not monotonic. So far this > is all I know. I do not know how to reproduce, etc. > > But when I look at this code, this looks really possible even > ignoring drivers/cpuidle/ which plays with update_ts_time_stats(). > > So, get_cpu_idle_time_us(last_update_time => NULL) does: > > if (ts->idle_active && !nr_iowait_cpu(cpu)) { > ktime_t delta = ktime_sub(now, ts->idle_entrytime); > > idle = ktime_add(ts->idle_sleeptime, delta); > } else { > idle = ts->idle_sleeptime; > } > > > Suppose that ts->idle_active == T. By the time we calculate > > idle = ktime_add(ts->idle_sleeptime, delta); > > this cpu can be already non-idle and ->idle_sleeptime can be already > updated by tick_nohz_stop_idle(), we return the wrong value. > > If user-space reads /proc/stat again after that, "idle" can obviously > go back. > > get_cpu_iowait_time_us() has the same problem. > > Plus nr_iowait_cpu() can change in between even if cpu stays idle, > io_schedule() can return on another CPU. > > Questions: > > - Any other reason why it can be non-monotonic? > > - Should we fix this or should we document that userspace > should handle this itself? > > IOW, is this is bug or not?
I don't know if we want to fix it (I personally think we should because it is not the first time I hear complains about this) but if we do, here is a possible fix:
https://lkml.org/lkml/2013/8/8/638
Thanks.
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