[LITMUS^RT] BUG: Dropped Tasks

[Glenn Elliott]

Hi Everyone,

I’ve run into a bug in Litmus, and I just wanted to pass it on.  I don’t have a fix for it yet.  Since we don’t have a formal bug tracking mechanism in place, I thought that I would just report it here.

Bug: Task is unlinked, but it is not put back on a ready queue.  As a result, the task is never rescheduled.  (This is under C-EDF, as well as Jeremy’s related C-FL scheduler.)

I first observed it over the winter break while gathering overhead measurements using Jonathan’s experiment-scripts.  The bug would happen while running a large task set of rtspin-based tasks.  The bug is rare.  It occurs maybe once every two hours on my test system (UNC’s ludwig).  I spent last weekend trying to reproduce the bug with TRACE() enabled.  Unfortunately, tracing seems to make the bug much less likely to occur.  I was only able to reproduce it once over many hours of task set execution.  Sadly, the TRACE buffer experienced an overflow when the bug tripped, so I couldn’t investigate further.  (This does imply that the bug may be related to heavy utilization since the ring buffer was prevented from emptying.)

This bug may or may not be related to the non-preemption bug that I reported last week.  I can’t say if using non-preemption made triggering the above bug more likely, or if last week’s bug is unrelated.

Thanks,
Glenn