[jdk16] RFR: 8259765: ZGC: Handle incorrect processor id reported by the operating system [v2]

[Per Liden]

> Some environments (e.g. OpenVZ containers) incorrectly report a logical processor id that is higher than the number of processors available. This is problematic, for example, when implementing CPU-local data structures, where the processor id is used to index into an array of length processor_count().
> 
> We've received crash reports from Jelastic (a Virtuozzo/OpenVZ user) where they run into this problem. We can workaround the problem in the JVM, until the underlying problem is fixed. Without this workaround ZGC can't be used in this environment.
> 
> This is currently a ZGC-specific issue, since ZGC is currently the only part of HotSpot that is using CPU-local data structures, but that could change in the future.
> 
> Just to clarify. In a Virtuozzo/OpenZV environment, it seems the underlying problem is not necessarily that sched_getcpu() returns an incorrect processor id, but rather that sysconf(_SC_NPROCESSORS_CONF) returns a too low number. Either way, sched_getcpu() and syconf(_SC_NPROCESSORS_CONF) seems to have different views of the world. This is not an issue in container environments such as Docker.
> 
> This patch works around this problem by letting os::processor_id() on Linux detect incorrect processor ids, and convert them to processor id 0. As mentioned in the comment in the code, this is safe, but not optimal for performance if the system actually has more than one processor. There's also a warning printed the first time this happen.
> 
> Testing: Manual testing with various fake/incorrect values returned from sched_getcpu().

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Review

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Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/124/files
  - new: https://git.openjdk.java.net/jdk16/pull/124/files/dbe8bd89..48d9b68a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=124&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=124&range=00-01

  Stats: 8 lines in 1 file changed: 2 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/124.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/124/head:pull/124

PR: https://git.openjdk.java.net/jdk16/pull/124