Have we changed /proc/stat idle statistics by NOHZ kernel?

[Michal Hocko]

Hi,
we have a customer reporting that /proc/stat doesn't provide correct
results about idle time if the machine is idle.
The issue is caused by the fact that tickles kernel doesn't update
kstat_cpu(i).cpustat.idle while it is tickles. Tools that parse this
file interpret the unchanged value as 0% idle since the last time.
While I personally do not think that measuring the idle machine is
that important one could say that the semantic of the file has changed
with NOHZ which is not good as we are trying to keep this interface
stable.
One way to fix this is to consider the current status of idle in
show_stat. The very primitive attempt of that can be seen bellow (on
top of the current Linus tree). I know it has several issue it just
illustrates what I am trying to say.  It will not work if jiffies
overflow while the CPU was tickles and it also misses locking and
handling !NOHZ configuration.

I have also noticed we have get_cpu_idle_time_us which should do
something similar. Should it be used instead or it is more intrusive?

Btw. is this considered to be a problem at all?

Thanks
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