Correctly calculate CPU percentages on linux with guests

[Foxlik]

Since guest is already included in user and guest_nice is included in nice, it got included twice in totals.

http://unix.stackexchange.com/questions/178045/proc-stat-is-guest-counted-into-user-time

Please doublecheck this, as I can test just on recent linux.

[giampaolo]

Uhm... good catch. I'm not completely certain the explanation given in that SO thread is correct though. The first message talks about times in /proc/stat (the system-wide CPU times) whereas who posted the first answer talks about /proc/[pid]/stat, which are process times.

[giampaolo]

I'm trying to find an official reference which states that guest and guest_nice in /proc/stats are already included in user time.

[Foxlik]

@giampaolo Good point. I think this is enough proof: https://github.com/torvalds/linux/blob/447976e/kernel/sched/cputime.c#L169

And for explanation see very last paragraph of https://lists.linuxfoundation.org/pipermail/virtualization/2009-August/013459.html

[giampaolo]

Thanks, that's enough. =)
After further investigation it seems we are also calculating the busy time wrong (and hence the percentage is wrong).
Right now it's done as busy = total - idle but according to htop (see here and here) it should be busy = total - idle - iowait.

To push this even further, I'm not sure whether we should adjust the guest time calculation only in cpu_percent() or also in cpu_times() (I mean subtracting the guest and guest_nice from user and nice).

[giampaolo]

This is now correctly fixed. Thanks for discovering the original issue. I had no idea the CPU percent related math was not 100% correct.

[Foxlik]

Great. Thanks!