ANDROID: binder: RT priority inheritance

Changes since v2 [1]:
- All patches in v2 not related to priority inheritance were merged, and hence removed from this series
- Fixed using the wrong mask in node scheduler policy calculation, originally reported by Ganesh Mahendran <[email protected]>
- Fixed using an uninitialized value for desired_prio, originally reported by Ganesh Mahendran <[email protected]>

Changes since v1 [2]:
- added more detailed commit messages and comments to the priority inheritance patches, including rationale for not using schet_setscheduler() directly, or rt_mutex prio inheritance. No functional changes.

[1]: https://lkml.kernel.org/r/[email protected]
[2]: https://lkml.kernel.org/r/[email protected]

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This patch series introduces support for priority inheritance of real-time scheduling policies in binder. With the introduction of Android Treble, functionality that used to be in a single process is now split over two or more processes, which communicate using binder IPC. For latency sensitive operations such as sensor events, Bluetooth audio and rendering, inheriting the (real-time) priority of the caller is crucial to meet requirements.

The implementation in this series directly calls into the scheduler to modify priorities, since I haven't found a way to make this work correctly with rt_mutex or other existing priority inheritance mechanisms. The main reasons a PI rt_mutex doesn't work well are the following:
1) Binder supports asynchronous transactions, where a caller isn't blocked on a result; therefore, the caller also couldn't block on a rt_mutex.
2) Binder supports the concept of 'node priority', where the priority of a call is not determined by the caller, but by the endpoint the caller calls in to.
3) There may not necessarily be any binder threads waiting to handle a transaction, so the caller doesn't always have a thread to change the priority of; instead, the first thread to pick up the work changes its own priority.
4) rt_mutex doesn't support non-RT policies (though a patch was sent to LKML once to address this).

More details in the patches themselves. I have found the current approach to be reliable, but I'm happy to look into suggestions to make this work with rt_mutex, or use other infrastructure.

All patches have already been reviewed by Android engineers and are merged in Android's common kernel trees.

Martijn Coenen (6):
ANDROID: binder: add support for RT prio inheritance.
ANDROID: binder: add min sched_policy to node.
ANDROID: binder: improve priority inheritance.
ANDROID: binder: add RT inheritance flag to node.
ANDROID: binder: don't check prio permissions on restore.
ANDROID: binder: Add tracing for binder priority inheritance.

 drivers/android/binder.c            | 294 ++++++++++++++++++++++++++++++++----
 drivers/android/binder_trace.h      |  24 +++
 include/uapi/linux/android/binder.h |  49 +++++-
 3 files changed, 334 insertions(+), 33 deletions(-)

2.15.0.rc2.357.g7e34df9404-goog