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

[Severin Gehwolf]

On Wed, 5 Nov 2025 18:13:55 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.

This pull request has now been integrated.

Changeset: 5d65c23c
Author:    Severin Gehwolf <sgehwolf at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/5d65c23cd99b72527dcfab9eb6da9510e7dc6330
Stats:     107 lines in 4 files changed: 90 ins; 0 del; 17 mod

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

Reviewed-by: cnorrbin, ayang, syan

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PR: https://git.openjdk.org/jdk/pull/28157