RFR: 8349988: Change cgroup version detection logic to not depend on /proc/cgroups [v4]

[Thomas Fitzsimmons]

On Mon, 31 Mar 2025 10:53:57 GMT, Ashutosh Mehra <asmehra at openjdk.org> wrote:

>> Thomas Fitzsimmons has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 13 additional commits since the last revision:
>> 
>>  - Merge branch 'master' into cgroups-v2-version-check-and-controllers-parsing-1
>>  - Replace literal tabs in procCgroupsCgroupsV1CpusetDisabledContent
>>  - Detect cpuset-disabled condition during cgroups v1 /proc/cgroups parsing
>>    
>>    Remove from cgroups v1 branch incorrect log messages about cpuset
>>    controller being optional.  Add test case for cgroups v1, cpuset
>>    disabled.
>>  - Improve !cgroups_v2_enabled branch comment
>>  - Debug-log optional and disabled cgroups v2 controllers
>>    
>>    Do not log enabled controllers that are not relevant to the JDK.
>>  - Move index declaration to scope in which it is used
>>  - Remove empty string check during cgroup.controllers parsing
>>  - Define ISSPACE_CHARS macro, use it in strsep call
>>  - Pass fgets result to strsep
>>  - Replace is_cgroupsV2 with cgroups_v2_enabled
>>    
>>    Also fix the testCgroupv1SystemdOnly and testCgroupv1NoMounts test
>>    cases such that their /proc/cgroups and /proc/self/cgroup contents
>>    correspond.  This prevents assertion failures these tests were
>>    producing when is_cgroupsV2 was replaced with cgroups_v2_enabled.
>>  - ... and 3 more: https://git.openjdk.org/jdk/compare/8b0ec7c1...b6926e15
>
> test/hotspot/jtreg/containers/cgroup/CgroupSubsystemFactory.java line 459:
> 
>> 457:     public void testCgroupv1SystemdOnly(WhiteBox wb) {
>> 458:         String procCgroups = cgroupv1CgInfoZeroHierarchy.toString();
>> 459:         String procSelfCgroup = cgroupV2SelfCgroup.toString();
> 
> I don't get why is this change required? The test name `testCgroupv1SystemdOnly` suggests it is testing cgroup v1 only but then it passes cgroup v2 proc file. Same for `testCgroupv1NoMounts`.

Thank you for reviewing.  This test consistency fix is discussed [here](https://github.com/openjdk/jdk/pull/23811#discussion_r1973877201) and [here](https://github.com/openjdk/jdk/pull/23811#discussion_r1978045429); I agree the result is confusing.  Instead I will change `cgroupv1CgInfoZeroHierarchy` to `cgroupv1CgInfoNonZeroHierarchy` which achieves the same effect using only `cgroup v1` fields.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23811#discussion_r2021787908