Friday, April 22, 2011

More RTEMS SMP Patches Coming

Some of you may be aware that SMP for RTEMS has been underway for about a year now.  The goal of the SMP effort is to have a simple, working, and correct implementation.  The first incarnation will have the following characteristics.
  • BSP SMP Interface definition with implementations for
    • PC386
    • LEON3
  • Simple SMP Aware Priority Based Scheduler
  • Faithful SMP safe version of RTEMS OS Critical Sections
    • Dispatch Disable
    • Interrupt Disable
  • Scheduler Simulator 
    • Test scenarios for new Simple SMP Scheduler
  • Features Not Present
    • Processor affinity
    • Deferred Floating Point context switch
    • Taking a core offline
The SMP implementation plan broken the effort into as many small steps as possible so it could be incrementally reviewed and merged.  This plan also allows for intermittent work.  This was critical due to the fact that all initial work was completely volunteer.  In addition to being planned as a series of small steps, the initial SMP implementation is focused on simplicity and correctness.  We can improve a simple working implementation.

Gedare Bloom and I made the first steps last summer when implemented the Pluggable Scheduler Framework for RTEMS and I added a "per cpu" data structure.  Together these allow us to provide an alternative scheduler that is SMP aware and to have the data required by RTEMS SuperCore to manage each core encapsulated and allocated properly.

Jennifer Averett and I have been working the past couple of months on completing the SMP support to RTEMS.   Jennifer and I are approaching a milestone of having a basic SMP system functional with the only major missing item being SMP safe interrupt disable sections.  We are about to file a set of PRs and merge our current work and it made sense to post a blog entry with status that the PRs could reference.
  • Test code - our test code is hacky since it has to force interprocessor interrupts. We need to integrate where these are generated   inside RTEMS.  Tests will be submitted once the code is in shape to work without "user-level" intervention.
  • Simple SMP Scheduler - implemented, tested with schedsim and our hacky test
  • Scheduler Simulator - multiple changes to improve its use during Scheduler development and to track changes in code base
  • PC386 BSP - SMP BSP support seems complete.
  • LEON3 BSP - SMP BSP support seems complete.
  • Context Switch Disable Critical Section - Working
Overall, (today) SMP RTEMS  can bring an SMP system out of reset, schedule across multiple cores, and command the first dispatch on the secondary cores.  The "disable dispatch" should be SMP safe now.  It can do this on pc386 and leon3.

The next major tasks are to integrate the generation of interprocessor interrupts for subsequent dispatch requests and system shutdown.  We also have to address interrupt disable SMP safety.

We are doing our best to break this into as many small incremental pieces as possible so the review and integration into the main tree is easier.

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