Setting thread's scheduling/priority at native (C/C++) side within .apk

[DAMIR LJUBIC]

Problem: I have native (C++) code within the apk.

Explicit setting scheduling/priority doesn’t have any effect.

Description: There is 3rd party code  (objective C), where all threads

will be created with POSIX APIs, specifying for the scheduling SCHED_RR and the corresponding priority.

This seems to have no effect, since my apk has no root privileges.

It seems that in implementation of the related POSIX APIs (like pthread_setschedparam), kernel checks for the CAP_SYS_NICE capability for any unprivileged process.

capabilities(7) - Linux manual page (man7.org)

Facts:

• SELinux currently running in permission mode
• The .apk is not root process, there is no init.rc, where this capability can be set – applied

Is there any way to overcome this issue?

@note Using JNI to attach the native thread - to create the Java thread, from which the 

priority (CFS: niceness) will be set (Process.setThreadPriority) is working, but in Java world there is no way to set real-time scheduling

https://godbolt.org/z/x5W4WjdY9

Thanks in advance,

Damir

[yu...@unity3d.com]

You can't set real-time scheduling in user space on non-rooted devices on Android.

setThreadPriority() is setting only niceness, that should be mentioned in the documentation.

that's all "by design". will let google folks speak but guess - security and safety being the reason, and so that the apps won't overuse this superpower.

Why would you need real-time scheduling for your thread?

Yury

[enh]

On Mon, Jan 2, 2023 at 11:23 PM 'yu...@unity3d.com' via android-ndk <andro...@googlegroups.com> wrote:

You can't set real-time scheduling in user space on non-rooted devices on Android.

setThreadPriority() is setting only niceness, that should be mentioned in the documentation.

that's all "by design". will let google folks speak but guess - security and safety being the reason, and so that the apps won't overuse this superpower.

yeah, "exactly what he said" :-)

funnily enough, when i inherited bionic it didn't report failures to set scheduler parameters from a pthread_create() call. i fixed that, and we found that basically everything failed because everyone was asking for real-time priority --- which they'd never actually been granted, but now those of them that were checking for errors were failing. i had to put in a hack that meant we'd continue to not report errors to any apk whose targetSdkVersion was lower than the new version.

 

Why would you need real-time scheduling for your thread?

Yury

On Tuesday, December 27, 2022 at 9:36:49 PM UTC+1 damir.m...@gmail.com wrote:

Problem: I have native (C++) code within the apk.

Explicit setting scheduling/priority doesn’t have any effect.

Description: There is 3rd party code  (objective C), where all threads

will be created with POSIX APIs, specifying for the scheduling SCHED_RR and the corresponding priority.

This seems to have no effect, since my apk has no root privileges.

It seems that in implementation of the related POSIX APIs (like pthread_setschedparam), kernel checks for the CAP_SYS_NICE capability for any unprivileged process.

capabilities(7) - Linux manual page (man7.org)

Facts:

• SELinux currently running in permission mode
• The .apk is not root process, there is no init.rc, where this capability can be set – applied

Is there any way to overcome this issue?

@note Using JNI to attach the native thread - to create the Java thread, from which the 

priority (CFS: niceness) will be set (Process.setThreadPriority) is working, but in Java world there is no way to set real-time scheduling

https://godbolt.org/z/x5W4WjdY9

Thanks in advance,

Damir

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