RFR: 8265836: OperatingSystemImpl.getCpuLoad() returns incorrect CPU load

[Severin Gehwolf]

On Fri, 23 Apr 2021 13:33:42 GMT, Hao Tang <github.com+7947546+tanghaoth90 at openjdk.org> wrote:

> OperatingSystemImpl.getCpuLoad() may return 1.0 in a container, even though the CPU load is obviously below 100%.
> 
> We created a 5-core container and run 4 "while (true)" loops in the container. OperatingSystemImpl.getCpuLoad() returned 1.0, which is incorrect (0.8 is correct).
> "systemLoad" in getCpuLoad() is exactly 4.0 before "systemLoad = Math.min(1.0, systemLoad);". The problem is caused by using the elapsed time (specified by "cpu.cfs_period_us") instead of the total CPU time (specified by "cpu.cfs_quota_us"). Therefore, it is more reasonable to divide cpu usage time by "quotaNanos" instead of "elapsedNanos".

@tanghaoth90 There are a couple of ways how CPU restrictions can be enforced. It appears that the current patch fixes the cpuacct controller case (`--cpu-quota`), but doesn't yet address `--cpu-shares` case. I've made a note of that in the bug.

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Changes requested by sgehwolf (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3656