RFR: JDK-8273526: Extend the OSContainer API pids controller with pids.current

[Severin Gehwolf]

On Thu, 9 Sep 2021 09:21:59 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

> https://bugs.openjdk.java.net/browse/JDK-8266490
> extended the OSContainer API in order to also support the pids controller of cgroups. However only pids.max output was added with 8266490.
> There is a second parameter pids.current , see https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#pid
> that would be helpful too and can be added to the OSContainer API .
> pids.current :
> A read-only single value file which exists on all cgroups.
> The number of processes currently in the cgroup and its descendants.
> 
> Best regards, Matthias

This could get simplified a bit as we don't need to consider `max` values for `pids.current`.

src/hotspot/os/linux/cgroupV1Subsystem_linux.cpp line 287:

> 285:  *    OSCONTAINER_ERROR for not supported
> 286:  */
> 287: jlong CgroupV1Subsystem::pids_current() {

`pids.current` never contains a string `max` (for unlimited). Therefore, we shouldn't need to do the `pids_current_val` -> `limit_from_str()` trick. We should be able to use `GET_CONTAINER_INFO(int, [...]` akin to `cpu_quota`. `int` or `long` would both be suitable. Up to you to decide which data type to use. I don't think it'll ever be beyond the maximum integrer value.

src/java.base/linux/classes/jdk/internal/platform/cgroupv1/CgroupV1Subsystem.java line 421:

> 419:     public long getPidsCurrent() {
> 420:         String pidsCurrentStr = CgroupSubsystemController.getStringValue(pids, "pids.current");
> 421:         return CgroupSubsystem.limitFromString(pidsCurrentStr);

`return getLongValue(pids, "pids.current");` should be sufficient here.

test/hotspot/jtreg/containers/docker/TestPids.java line 97:

> 95:                 System.out.println("DEBUG: parts.length = " + parts.length);
> 96:                 if (expectedValue.equals("no_value_expected")) {
> 97:                     System.out.println("No value expected for " + lineMarker);

Perhaps this debug print could say `System.out.println("Found '" + lineMarker + "' with value: '" + <value> +"');` and actually parse the number as we'd expect an integer there?

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Changes requested by sgehwolf (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5437