RFR: 8336881: [Linux] Support for hierarchical limits for Metrics [v2]

[Severin Gehwolf]

On Mon, 29 Jul 2024 14:18:24 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> 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 three commits:
> 
>  - 8336881: [Linux] Support for hierarchical limits for Metrics
>  - Unify 4 copies of adjust_controller()
>  - Fix a needless whitespace change

Moving to draft. Needs the hotspot fix which gets a reboot.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/20280#issuecomment-2296180545