Tuesday, March 8, 2011

Coverage Testing Finds Unexpected Bug

Recent posts have been about debugging experiences. Jennifer and I have been working on a simple implementation of the RTEMS priority based scheduler to illustrate how to write an alternative scheduler. The primary RTEMS Scheduler is deterministic (e.g. constant, predictable) in performance because it uses a FIFO per priority and a two level bit map to assist in look ups. The simple scheduler uses a single list for all tasks and searches down the list to perform inserts. Its worst case is thus O(n) where n is the number of ready tasks. But this post is above debugging, not about the new scheduler. We will discuss that another time.

Jennifer and I want the simple scheduler to have 100% test coverage when it is merged. This has required us to run coverage analysis on RTEMS and check the results. We haven't yet merged this code so it isn't showing up in published runs. She noticed that thee method _Scheduler_priority_Enqueue_first was reported as never executed. We both found this incredulous since this is called as part of inheriting a priority. We knew this was well tested. She was perplexed and asked me what I thought. It turned out to be a recently introduced typo where _Scheduler_priority_Enqueue was configured as the handler for both the enqueue and enqueue entries points in the Scheduler Priority table. This also explained a couple of test failures Jennifer had noticed but hadn't investigated yet. This is the pertinent part of patch:

- _Scheduler_priority_Enqueue, /* enqueue_first entry point */ \
+ _Scheduler_priority_Enqueue_first, /* enqueue_first entry point */ \

Again, this problem was highlighted by an unexpected drop in the coverage results. RTEMS has very good test coverage and an unexpected change can indicate a problem. The fact that this method was never called was a huge hint to the cause. The answer popped almost instantly in my head. Debugging the test failures would almost certainly have taken much longer.

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