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

[Thomas Fitzsimmons]

On Mon, 31 Mar 2025 10:53:50 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/eb4ea706...b6926e15
>
> src/hotspot/os/linux/cgroupSubsystem_linux.cpp line 81:
> 
>> 79:   // file system magic.  If it does not then heuristics are required to determine
>> 80:   // if cgroups v1 is usable or not.
>> 81:   if (statfs(sys_fs_cgroup, &fsstat) != -1) {
> 
> I feel this logic should be moved to `determine_type` as it is responsible for determining the version of the cgroup subsystem.

OK, I tend to agree; I will investigate alternatives.  I did consider putting the `statfs` logic inside but ended up leaving it outside because `determine_type` is called by the `whitebox` framework, and "mocking" `statfs` is not possible with regular files.  The idea is to allow the test suite to simply mock the `statfs` result via the boolean `cgroups_v2_enabled` argument.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23811#discussion_r2021793827