RFR: 8336881: [Linux] Support for hierarchical limits for Metrics [v8]

[Severin Gehwolf]

> Please review this fix for cgroups-based metrics reporting in the `jdk.internal.platform` package. This fix is supposed to address wrong reporting of certain limits if the limits aren't set at the leaf nodes.
> 
> For example, on cg v2, the memory limit interface file is `memory.max`. Consider a cgroup path of  `/a/b/c/d`. The current code only reports the limits (via Metrics) correctly if it's set at `/a/b/c/d/memory.max`. However, some systems - like a systemd slice - sets those limits further up the hierarchy. For example at `/a/b/c/memory.max`. `/a/b/c/d/memory.max` might be set to the value `max` (for unlimited), yet `/a/b/c/memory.max` would report the actual limit value (e.g. `1048576000`).
> 
> This patch addresses this issue by:
> 
> 1. Refactoring the interface lookup code to relevant controllers for cpu/memory. The CgroupSubsystem classes then delegate to those for the lookup. This facilitates having an API for the lookup of an updated limit in step 2.
> 2. Walking the full hierarchy of the cgroup path (if any), looking for a lower limit than at the leaf. Note that it's not possible to raise the limit set at a path closer to the root via the interface file at a further-to-the-leaf-level. The odd case out seems to be `max` values on some systems (which seems to be the default value).
> 
> As an optimization this hierarchy walk is skipped on containerized systems (like K8S), where the limits are set in interface files at the leaf nodes of the hierarchy. Therefore there should be no change on those systems.
> 
> This patch depends on the Hotspot change implementing the same for the JVM so that `Metrics.isContainerized()` works correctly on affected systems where `-XshowSettings:system` currently reports `System not containerized` due to the missing JVM fix. A test framework for such hierarchical systems has been proposed in [JDK-8333446](https://bugs.openjdk.org/browse/JDK-8333446). This patch adds a test using that framework among some simpler unit tests.
> 
> Thoughts?
> 
> Testing:
> 
> - [x] GHA
> - [x] Container tests on Linux x86_64 on cg v1 and cg v2 systems
> - [x] Some manual testing using systemd slices

Severin Gehwolf has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 32 commits:

 - Merge branch 'master' into jdk-8336881-metrics-systemd-slice
 - Improve systemd slice test for non-root on cg v2
 - Fix unit tests
 - Add JVM_HostActiveProcessorCount using JVM api
 - Merge branch 'jdk-8322420_cgroup_hierarchy_walk_init' into jdk-8336881-metrics-systemd-slice
 - Merge branch 'master' into jdk-8322420_cgroup_hierarchy_walk_init
 - Merge branch 'jdk-8322420_cgroup_hierarchy_walk_init' into jdk-8336881-metrics-systemd-slice
 - Merge branch 'master' into jdk-8322420_cgroup_hierarchy_walk_init
 - Adjust test and fix code to return lowest hierarchical limit
 - Merge branch 'jdk-8322420_cgroup_hierarchy_walk_init' into jdk-8336881-metrics-systemd-slice
 - ... and 22 more: https://git.openjdk.org/jdk/compare/d9fdf69c...0f559199

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Changes: https://git.openjdk.org/jdk/pull/20280/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20280&range=07
  Stats: 1595 lines in 26 files changed: 1344 ins; 152 del; 99 mod
  Patch: https://git.openjdk.org/jdk/pull/20280.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20280/head:pull/20280

PR: https://git.openjdk.org/jdk/pull/20280