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?
Oleg.
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