ThreadPriorityPolicy settings for non-root users

[Baesken, Matthias]

> 
> Have you got real use cases for this?
>

Hi David, yes  we have a user  who  was setting  the "infamous"   ThreadPriorityPolicy=2   (+ then different prios/niceness levels  for  different threads) ,  but that is now no longer working in current JDK11  .

Best regards, Matthias


> -----Original Message-----
> From: David Holmes <david.holmes at oracle.com>
> Sent: Freitag, 21. Dezember 2018 11:56
> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
> Subject: Re: ThreadPriorityPolicy settings for non-root users
> 
> On 21/12/2018 6:39 pm, Baesken, Matthias wrote:
> > Hi  David  ,   it might be  that  the functionality  is  seen as not very helpful by
> some and removed  or deprecated  in  some future  Java release.
> >
> > However it is present in the current JDKs and should work there nicely .
> 
> Sorry but that's a bit naive. The code is old and bit-rotted and in some
> cases (Mac port) likely never used, so the idea that "it's there so it
> should work" is just not realistic - sorry.
> 
> > Currently  I have some points I do not like about the current state :
> >
> > - the root-check  is not consistent , it is present  on Linux /  BSD (Mac)     but
> I don't see it on Solaris
> 
> Wasn't needed on Solaris. User-level capabilities sufficed.
> 
> > - It  ignores currently  the  CAP_SYS_NICE capability
> 
> It never supported it. AFAIK the linux code doesn't really support any
> capability based permissions.
> 
> > - it ignores that  setting a  higher niceness works nicely  on most OS
> (checked Linux/Solaris/BSD)  without being root  (or having special
> capabilities)
> 
> The priority control was never really about tweaking niceness levels.
> 
> > -  the root check makes testing hard  (maybe that's why the Mac version
> was a bit broken?)
> 
> Running under sudo isn't that hard.
> 
> Sorry I'm not very supportive here - this isn't something that needs
> some minor tweaking to bring back online, it's something that may never
> have worked well in the first place.
> 
> Have you got real use cases for this?
> 
> Cheers,
> David
> 
> >
> > Best regards, Matthias
> >
> >
> >> -----Original Message-----
> >> From: David Holmes <david.holmes at oracle.com>
> >> Sent: Donnerstag, 20. Dezember 2018 12:06
> >> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
> >> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
> >> Subject: Re: ThreadPriorityPolicy settings for non-root users
> >>
> >> Hi Matthias,
> >>
> >> The more I think about this the more I see it as a huge can of worms.
> >> There are very, very, limited usecases for managing the priority of
> >> individual threads within a running Java application. Adjusting the
> >> process priority/nice-ness is effective and much simpler.
> >>
> >> Cheers,
> >> David
> >>
> >> On 20/12/2018 3:13 am, Baesken, Matthias wrote:
> >>>
> >>> Hello ,
> >>>     currently OpenJDK supports 2 ThreadPriorityPolicy settings,  0
> (normal,
> >> the default) and 1,
> >>> the so called "aggressive" mode :
> >>>
> >>> "1 : Aggressive.                                                 "\
> >>> "    Java thread priorities map over to the entire range of      "\
> >>> "    native thread priorities. Higher Java thread priorities map "\
> >>> "    to higher native thread priorities. This policy should be   "\
> >>> "    used with care, as sometimes it can cause performance       "\
> >>> "    degradation in the application and/or the entire system. On "\
> >>> "    Linux this policy requires root privilege.")                 \
> >>>
> >>> Currently  we check directly for root in os_bsd.cpp and os_linux.cpp (the
> >> text  in globals.hpp mentions only Linux which seems to be not fully
> correct):
> >>>
> >>> if (geteuid() != 0) { ... } in function prio_init().
> >>>
> >>> (looks like the check is not done for other platforms).
> >>>
> >>> However the check for root (e.g. on Linux)  hinders users to set a
> >> ****lower priority**** for a thread (== increase the "niceness" level)
> >>> when running as a non-root user  (there might be strange ways from
> >> outside the VM with calling scripts and renice but .... ).
> >>>
> >>>
> >>> In older JDKs (e.g. JDK8) there was a "workaround" to use for example
> >> ThreadPriorityPolicy=2 to avoid the root-check,
> >>> but this is not possible any more in recent JDKs (10/11) after the range
> >> check (0,1) has been introduced for the ThreadPriorityPolicy flag
> >>> (and probably the old workaround was not a good one anyway because
> it
> >> was undocumented).
> >>>
> >>> So do you think we could introduce another XX-flag  (
> >> AllowAggressiveThreadPriorityPolicyForAllUsers, or some better name)
> >>> that allows using the "aggressive" mode for non-root users ? Another
> >> option would be to add another mode 2 for ThreadPriorityPolicy
> >>> that documents the behavior (like mode 1 but without root-user check).
> >>>
> >>> If I get it right, even  setting  ***higher prios*** (lower niceness) is
> >> possible for non-root users on systems configured in an appropriate way
> >>> (using the CAP_SYS_NICE capability).
> >>>
> >>> But setting  lower prio / higher niceness is even possible for normal users
> >> NOW without special config, it is just disabled by the root-check
> >>> which is very bad.
> >>>
> >>> Best regards, Matthias
> >>>