"Currently in mainline the balancing of multiple RT threads is quite broken. That is to say that a high priority thread that is scheduled on a CPU with a higher priority thread, may need to unnecessarily wait while it can easily run on another CPU that's running a lower priority thread," began Steven Rostedt, describing his patchset to introduce improved real time task balancing. He explained:

"Balancing (or migrating) tasks in general is an art. Lots of considerations must be taken into account. Cache lines, NUMA and more. This is true with general processes which expect high through put and migration can be done in batch. But when it comes to RT tasks, we really need to put them off to a CPU that they can run on as soon as possible. Even if it means a bit of cache line flushing. Right now an RT task can wait several milliseconds before it gets scheduled to run. And perhaps even longer. The migration thread is not fast enough to take care of RT tasks."

Steven described his test cases and numerous issues he noticed with the current balancing code, noting, "to solve this issue, I've changed the RT task balancing from a passive method (migration thread) to an active method. This new method is to actively push or pull RT tasks when they are woken up or scheduled."

Ingo Molnar announced that the real time patchset [story] that he and Thomas Gleixner maintain is now available as a series of 374 broken out patches, "from now on (as of 2.6.22.1-rt2) it will be part of every upstream -rt release and it is available from the -rt download site". Regarding the patches, he notes that it's responsible for, "698 files changed, 27920 insertions(+), 9603 deletions(-)", going on to note, "which is impressive as we moved a huge chunk of -rt into mainline already ;-) The series file is attached below.". Ingo explains:

"the purpose of this finegrained splitup is to foster (even ;-) quicker upstream integration of various -rt features, and to see the full -rt tree integrated upstream. We also hope that this split-up queue helps various vendors standardize their (currently quite splintered) real-time implementations to the upstream -rt patchset. The queue is not (yet) bisectable at every point, and many of the splits are thematic, to allow the simpler future handling of updates."