proc/stat: idle goes backward

[Oleg Nesterov <oleg@redhat.com>]

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.