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

[Severin Gehwolf]

On Tue, 1 Apr 2025 13:39:53 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> @tstuefe @ashu-mehra Could you please help with a second review?
>
>> @jerboaa @fitzsim Does the current mainline code handles mixed configuration where in some controllers are v1 and others v2? For example cpu controller is mounted as v1 while memory controller as v2. If yes, does this patch continue to support such configuration?
> 
> The current code does not allow mixed configuration for "relevant" controllers (essentially cpu and memory). That is, they ought to be v1 or v2. In the hybrid case (systemd running on unified), it's considered v1. I don't think this patch changes any of it.

> @jerboaa @fitzsim, is there any plan to backport this fix (to 21)?

Eventually yes. But this change hasn't seen a lot of real-world exposure yet. It would be good to have that before attempting a backport.

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PR Comment: https://git.openjdk.org/jdk/pull/23811#issuecomment-2862216668