ThreadPriorityPolicy settings for non-root users

[David Holmes]

On 21/12/2018 9:09 pm, Baesken, Matthias wrote:
>>
>> 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  .

I'd be interested to know what the actual usecase is. But I think a 
native method would be the best way to handle it.

David

> 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
>>>>>