RFR: 8370492: [Linux] Update cpu shares to cpu.weight mapping function [v5]

[Albert Mingkun Yang]

On Wed, 12 Nov 2025 20:28:41 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> Please review this change to the Linux container detection code updating the mapping function of cg v2 to most recent runtimes' implementation. OCI has been standardized with cgroup v1 in mind and there is a mapping function in place - implemented by container runtimes such as `runc` or `crun` - which map the passed `--cpu-shares` value on the engine's `run` command to the internal cgroup v2 `cpu.weight` interface file's values.
>> 
>> For runtimes lower than 1.23 (crun) and 1.3.2 (runc) the mapping function was (where `f(x)=<cpu.weight value>` and `x=<cpu-shares CLI value>`):
>> 
>> 
>> f(x) = {(x - 2) \times 9999 + 1 \over 262142}
>> 
>> 
>> This changed in `crun` >= 1.23 and `runc` >= 1.3.2 to:
>> 
>> 
>> f(x)=10^{({{\log_{2}(x)}^2 \over 612} + {125 \over 612} \times \log_{2}(x) - {7 \over 34})}
>> 
>> 
>> Therefore, we need to find the inverse of the new quadratic mapping function and use that over the inverse of the old mapping function. Since the input domain is `[2,262144]` for `--cpu-shares` mapping to `[1,10000]` for `cpu.weight` we can discount the negative exponent value as a possible solution.
>> 
>> Since there are production systems out there with those runtimes already I believe it's time to change the mapping function internal to the JDK to the new one in JDK 26. There is a chance that JDK 26 would run on old runtimes in a container using cgroup v2, resulting in wrong diagnostics. The chance of this happening will decrease over time.
>> 
>> Thoughts?
>> 
>> **Testing:**
>> - [x] GHA
>> - [x] `test/hotspot/jtreg/containers` and `test/jdk/jdk/internal/platform` on cgroup v1 (with an old runtime - code not affected) and on cgroup v2 with a new runtime. All tests passed.
>
> Severin Gehwolf has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix TestMisc for podman on cg v1 in rootless mode.

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28157#pullrequestreview-3464591706