RFR : 8215962: Support ThreadPriorityPolicy mode 1 for non-root users on linux/bsd

[Baesken, Matthias]

Hello , please  review the following patch .

Currently, when ThreadPriorityPolicy is set to 1 (so called "Aggressive mode"), on linux and bsd(+Mac) a root-user-check (geteuid() != 0)) is done.
See for example the coding in jdk/src/hotspot/os/linux/os_linux.cpp int prio_init().

However the root-user-check has a few drawbacks:
- it blocks the capabilities feature available on current Linux distros (CAP_SYS_NICE capability) that can be used to allow setting lower niceness also for non-root
- setting a higher "niceness" (lower priority) is not possible on Linux for non-root because of the geteuid check

We  had a discussion about this in "ThreadPriorityPolicy settings for non-root users"  ,  with this suggestion :

https://mail.openjdk.java.net/pipermail/hotspot-dev/2018-December/035986.html

....

> Just drop the root check for ThreadPriorityPolicy=1 and let the underlying system

> permissions control success or failure.

I  did  the change  in this  webrev :


Bug/webrev :

https://bugs.openjdk.java.net/browse/JDK-8215962

http://cr.openjdk.java.net/~mbaesken/webrevs/8215962.0/


Best Regards ,  Matthias