Understanding Priority Inversion

Excerpted from: http://www.embedded.com/electronics-blogs/beginner-s-corner/4023947/Introduction-to-Priority-Inversion

Priority inversions

The real trouble arises at run-time, when a medium-priority task preempts a lower-priority task using a shared resource on which the higher-priority task is pending. If the higher-priority task is otherwise ready to run, but a medium-priority task is currently running instead, a priority inversion is said to occur.

Figure 1 Priority inversion timeline

This dangerous sequence of events is illustrated in Figure 1. Low-priority Task L and high-priority Task H share a resource. Shortly after Task L takes the resource, Task H becomes ready to run. However, Task H must wait for Task L to finish with the resource, so it pends. Before Task L finishes with the resource, Task M becomes ready to run, preempting Task L. While Task M (and perhaps additional intermediate-priority tasks) runs, Task H, the highest-priority task in the system, remains in a pending state.

Many priority inversions are innocuous or, at most, briefly delay a task that should run right away. But from time to time a system-critical priority inversion takes place. Such an event occurred on the Mars Pathfinder mission in July 1997. The Pathfinder mission is best known for the little rover that took high-resolution color pictures of the Martian surface and relayed them back to Earth.

The problem was not in the landing software, but in the mission software run on the Martian surface. In the spacecraft, various devices communicated over a MIL-STD-1553 data bus. Activity on this bus was managed by a pair of high-priority tasks. One of the bus manager tasks communicated through a pipe with a low-priority meteorological science task.

On Earth, the software mostly ran without incident. On Mars, however, a problem developed that was serious enough to trigger a series of software resets during the mission. The sequence of events leading to each reset began when the low-priority science task was preempted by a couple of medium-priority tasks while it held a mutex related to the pipe. While the low-priority task was preempted, the high-priority bus distribution manager tried to send more data to it over the same pipe. Because the mutex was still held by the science task, the bus distribution manager was made to wait. Shortly thereafter, the other bus scheduler became active. It noticed that the distribution manager hadn't completed its work for that bus cycle and forced a system reset.

This problem was not caused by a mistake in the operating system, such as an incorrectly implemented semaphore, or in the application. Instead, the software exhibited behavior that is a known "feature" of semaphores and intertask communication. In fact, the RTOS used on Pathfinder featured an optional priority-inversion workaround; the scientists at JPL simply hadn't been aware of that option. Fortunately, they were able to recreate the problem on Earth, remotely enable the workaround, and complete the mission successfully.

Workarounds

Research on priority inversion has yielded two solutions. The first is called priority inheritance. This technique mandates that a lower-priority task inherit the priority of any higher-priority task pending on a resource they share. This priority change should take place as soon as the high-priority task begins to pend; it should end when the resource is released. This requires help from the operating system.

The second solution, priority ceilings, associates a priority with each resource; the scheduler then transfers that priority to any task that accesses the resource. The priority assigned to the resource is the priority of its highest-priority user, plus one. Once a task finishes with the resource, its priority returns to normal.

A beneficial feature of the priority ceiling solution is that tasks can share resources simply by changing their priorities, thus eliminating the need for semaphores:

void TaskA(void)
{
 ...
 SetTaskPriority(RES_X_PRIO);
 // Access shared resource X.
 SetTaskPriority(TASK_A_PRIO);
 ...
}

While Task A's priority is elevated (and it is accessing shared resource X), it should not pend on any other resource. The higher-priority user will only become the highest-priority ready task when the lower-priority task is finished with their shared resource.

While not all of us are writing software for missions to Mars, we should learn from past mistakes and implement solutions that don't repeat them. Many commercial RTOSes include support for either priority inheritance or priority ceilings. Just make sure you enable one.

Date: Fri, 9 Jan 1998 14:13:58 -0800
From: Mike Jones 
Subject: Re: What really happened on Mars? by Glenn Reeves (RISKS-19.49)

> Date: Monday, December 15, 1997 10:28 AM
> From: Glenn E Reeves 
> Subject:      Re: [Fwd: FW: What really happened on Mars?]
>
> What really happened on Mars ?
>
>By now most of you have read Mike's (mbj@microsoft.com) summary of Dave
>Wilner's comments given at the IEEE Real-Time Systems Symposium.  I don't
>know Mike and I didn't attend the symposium (though I really wish I had now)
>and I have not talked to Dave Wilner since before the talk.  However, I did
>lead the software team for the Mars Pathfinder spacecraft.  So, instead of
>trying to find out what was said I will just tell you what happened.  You
>can make your own judgments.
>
>I sent this message out to everyone who was a recipient of Mike's original
>that I had an e-mail address for.  Please pass it on to anyone you sent the
>first one to.  Mike, I hope you will post this wherever you posted the
>original.
>
>Since I want to make sure the problem is clearly understood I need to step
>through each of the areas which contributed to the problem.
>
>THE HARDWARE
>
>The simplified view of the Mars Pathfinder hardware architecture looks like
>this.  A single CPU controls the spacecraft.  It resides on a VME bus which
>also contains interface cards for the radio, the camera, and an interface to
>a 1553 bus.  The 1553 bus connects to two places : The "cruise stage" part
>of the spacecraft and the "lander" part of the spacecraft.  The hardware on
>the cruise part of the spacecraft controls thrusters, valves, a sun sensor,
>and a star scanner.  The hardware on the lander part provides an interface
>to accelerometers, a radar altimeter, and an instrument for meteorological
>science known as the ASI/MET.  The hardware which we used to interface to
>the 1553 bus (at both ends) was inherited from the Cassini spacecraft.  This
>hardware came with a specific paradigm for its usage : the software will
>schedule activity at an 8 Hz rate.  This **feature** dictated the
>architecture of the software which controls both the 1553 bus and the
>devices attached to it.
>
>THE SOFTWARE ARCHITECTURE
>
>The software to control the 1553 bus and the attached instruments was
>implemented as two tasks.  The first task controlled the setup of
>transactions on the 1553 bus (called the bus scheduler or bc_sched task) and
>the second task handled the collection of the transaction results i.e. the
>data.  The second task is referred to as the bc_dist (for distribution)
>task.  A typical timeline for the bus activity for a single cycle is shown
>below.  It is not to scale.  This cycle was constantly repeated.
>
>     |< ------------- .125 seconds ------------------------>|
>
>     |<***************|                    |********|   |**>|
>
>                      |<- -="" active="" bc_dist="">|    bc_sched active
>     |< -bus active ->|                             |<->|
>
>
> ----|----------------|--------------------|--------|---|---|-------
>     t1               t2                   t3       t4  t5  t1
>
>The *** are periods when tasks other than the ones listed are executing.
>Yes, there is some idle time.
>
>t1 - bus hardware starts via hardware control on the 8 Hz boundary. The
>transactions for the this cycle had been set up by the previous execution of
>the bc_sched task.
>t2 - 1553 traffic is complete and the bc_dist task is awakened.
>t3 - bc_dist task has completed all of the data distribution
>t4 - bc_sched task is awakened to setup transactions for the next cycle
>t5 - bc_sched activity is complete
>
>The bc_sched and bc_dist tasks check each cycle to be sure that the other
>had completed its execution.  The bc_sched task is the highest priority task
>in the system (except for the vxWorks "tExec" task).  The bc_dist is third
>highest (a task controlling the entry and landing is second).  All of the
>tasks which perform other spacecraft functions are lower.  Science

>functions, such as imaging, image compression, and the ASI/MET task are
>still lower.
>
>Data is collected from devices connected to the 1553 bus only when they are
>powered.  Most of the tasks in the system that access the information
>collected over the 1553 do so via a double buffered shared memory mechanism
>into which the bc_dist task places the latest data.  The exception to this
>is the ASI/MET task which is delivered its information via an interprocess
>communication mechanism (IPC).  The IPC mechanism uses the vxWorks pipe()
>facility.  Tasks wait on one or more IPC "queues" for messages to arrive.
>Tasks use the select() mechanism to wait for message arrival.  Multiple
>queues are used when both high and lower priority messages are required.
>Most of the IPC traffic in the system is not for the delivery of real-time
>data.  However, again, the exception to this is the use of the IPC mechanism
>with the ASI/MET task.  The cause of the reset on Mars was in the use and
>configuration of the IPC mechanism.
>
>THE FAILURE
>
>The failure was identified by the spacecraft as a failure of the bc_dist
>task to complete its execution before the bc_sched task started.  The
>reaction to this by the spacecraft was to reset the computer.  This reset
>reinitializes all of the hardware and software. It also terminates the
>execution of the current ground commanded activities.  No science or
>engineering data is lost that has already been collected (the data in RAM is
>recovered so long as power is not lost).  However, the remainder of the
>activities for that day were not accomplished until the next day.
>
>The failure turned out to be a case of priority inversion (how we discovered
>this and how we fixed it are covered later).  The higher priority bc_dist
>task was blocked by the much lower priority ASI/MET task that was holding a
>shared resource.  The ASI/MET task had acquired this resource and then been
>preempted by several of the medium priority tasks.  When the bc_sched task
>was activated, to setup the transactions for the next 1553 bus cycle, it
>detected that the bc_dist task had not completed its execution.  The
>resource that caused this problem was a mutual exclusion semaphore used
>within the select() mechanism to control access to the list of file
>descriptors that the select() mechanism was to wait on.
>
>The select mechanism creates a mutual exclusion semaphore to protect the
>"wait list" of file descriptors for those devices which support select.  The
>vxWorks pipe() mechanism is such a device and the IPC mechanism we used is
>based on using pipes. The ASI/MET task had called select, which had called
>pipeIoctl(), which had called selNodeAdd(), which was in the process of
>giving the mutex semaphore.  The ASI/ MET task was preempted and semGive()
>was not completed.  Several medium priority tasks ran until the bc_dist task
>was activated.  The bc_dist task attempted to send the newest ASI/MET data
>via the IPC mechanism which called pipeWrite().  pipeWrite() blocked, taking
>the mutex semaphore.  More of the medium priority tasks ran, still not
>allowing the ASI/MET task to run, until the bc_sched task was awakened.  At
>that point, the bc_sched task determined that the bc_dist task had not
>completed its cycle (a hard deadline in the system) and declared the error
>that initiated the reset.
>
>HOW WE FOUND IT
>
>The software that flies on Mars Pathfinder has several debug features within
>it that are used in the lab but are not used on the flight spacecraft (not
>used because some of them produce more information than we can send back to
>Earth).  These features were not "fortuitously" left enabled but remain in
>the software by design.  We strongly believe in the "test what you fly and
>fly what you test" philosophy.
>
>One of these tools is a trace/log facility which was originally developed to
>find a bug in an early version of the vxWorks port (Wind River ported
>vxWorks to the RS6000 processor for us for this mission).  This trace/log

>facility was built by David Cummings who was one of the software engineers
>on the task.  Lisa Stanley, of Wind River, took this facility and
>instrumented the pipe services, msgQ services, interrupt handling, select
>services, and the tExec task.  The facility initializes at startup and
>continues to collect data (in ring buffers) until told to stop.  The
>facility produces a voluminous dump of information when asked.
>
>After the problem occurred on Mars we did run the same set of activities
>over and over again in the lab.  The bc_sched was already coded so as to
>stop the trace/log collection and dump the data (even though we knew we
>could not get the dump in flight) for this error.  So, when we went into the
>lab to test it we did not have to change the software.
>
>In less that 18 hours we were able to cause the problem to occur. Once we
>were able to reproduce the failure the priority inversion problem was
>obvious.
>
>HOW WAS THE PROBLEM CORRECTED
>
>Once we understood the problem the fix appeared obvious : change the
>creation flags for the semaphore so as to enable the priority inheritance.
>The Wind River folks, for many of their services, supply global
>configuration variables for parameters such as the "options" parameter for
>the semMCreate used by the select service (although this is not documented
>and those who do not have vxWorks source code or have not studied the source
>code might be unaware of this feature).  However, the fix is not so obvious
>for several reasons :
>
>1) The code for this is in the selectLib() and is common for all device
>creations.  When you change this global variable all of the select
>semaphores created after that point will be created with the new options.
>There was no easy way in our initialization logic to only modify the
>semaphore associated with the pipe used for bc_dist task to ASI/MET task
>communications.
>
>2) If we make this change, and it is applied on a global basis, how will
>this change the behavior of the rest of the system ?
>
>3) The priority inversion option was deliberately left out by Wind River in
>the default selectLib() service for optimum performance.  How will
>performance degrade if we turn the priority inversion on ?
>
>4) Was there some intrinsic behavior of the select mechanism itself that
>would change if the priority inversion was enabled ?
>
>We did end up modifying the global variable to include the priority
>inversion.  This corrected the problem.  We asked Wind River to analyze the
>potential impacts for (3) and (4). They concluded that the performance
>impact would be minimal and that the behavior of select() would not change
>so long as there was always only one task waiting for any particular file
>descriptor.  This is true in our system.  I believe that the debate at Wind
>River still continues on whether the priority inversion option should be on
>as the default.  For (1) and (2) the change did alter the characteristics of
>all of the select semaphores.  We concluded, both by analysis and test, that
>there was no adverse behavior.  We tested the system extensively before we
>changed the software on the spacecraft.
>
>HOW WE CHANGED THE SOFTWARE ON THE SPACECRAFT
>
>No, we did not use the vxWorks shell to change the software (although the
>shell is usable on the spacecraft).  The process of "patching" the software
>on the spacecraft is a specialized process.  It involves sending the
>differences between what you have onboard and what you want (and have on
>Earth) to the spacecraft.  Custom software on the spacecraft (with a whole
>bunch of validation) modifies the onboard copy.  If you want more info you
>can send me e-mail.
>
>WHY DIDN'T WE CATCH IT BEFORE LAUNCH ?
>
>The problem would only manifest itself when ASI/MET data was being collected
>and intermediate tasks were heavily loaded.  Our before launch testing was
>limited to the "best case" high data rates and science activities.  The fact
>that data rates from the surface were higher than anticipated and the amount

>of science activities proportionally greater served to aggravate the
>problem.  We did not expect nor test the "better than we could have ever
>imagined" case.
>
>HUMAN NATURE, DEADLINE PRESSURES
>
>We did see the problem before landing but could not get it to repeat when we
>tried to track it down.  It was not forgotten nor was it deemed unimportant.
>
>Yes, we were concentrating heavily on the entry and landing software.  Yes,
>we considered this problem lower priority.  Yes, we would have liked to have
>everything perfect before landing.  However, I don't see any problem here
>other than we ran out of time to get the lower priority issues completed.
>
>We did have one other thing on our side; we knew how robust our system was
>because that is the way we designed it.
>
>We knew that if this problem occurred we would reset.  We built in
>mechanisms to recover the current activity so that there would be no
>interruptions in the science data (although this wasn't used until later in
>the landed mission).  We built in the ability (and tested it) to go through
>multiple resets while we were going through the Martian atmosphere.  We
>designed the software to recover from radiation induced errors in the memory
>or the processor.  The spacecraft would have even done a 60 day mission on
>its own, including deploying the rover, if the radio receiver had broken
>when we landed.  There are a large number of safeguards in the system to
>ensure robust, continued operation in the event of a failure of this type.
>These safeguards allowed us to designate problems of this nature as lower
>priority.
>
>We had our priorities right.
>
>ANALYSIS AND LESSONS
>
>Did we (the JPL team) make an error in assuming how the select/pipe
>mechanism would work ?  Yes, probably.  But there was no conscious decision
>to not have the priority inversion enabled.  We just missed it.  There are
>several other places in the flight software where similar protection is
>required for critical data structures and the semaphores do have priority
>inversion protection.  A good lesson when you fly COTS stuff - make sure you
>know how it works.
>
>Mike is quite correct in saying that we could not have figured this out
>**ever** if we did not have the tools to give us the insight.  We built many
>of the tools into the software for exactly this type of problem.  We always
>planned to leave them in.  In fact, the shell (and the stdout stream) were
>very useful the entire mission.  If you want more detail send me a note.
>
>SETTING THE RECORD STRAIGHT
>
>First, I want to make sure that everyone understands how I feel in regard to
>Wind River.  These folks did a fantastic job for us.  They were enthusiastic
>and supported us when we came to them and asked them to do an affordable
>port of vxWorks.  They delivered the alpha version in 3 months.  When we had
>a problem they put some of the brightest engineers I have ever worked with
>on the problem.  Our communication with them was fantastic.  If they had not
>done such a professional job the Mars Pathfinder mission would not have been
>the success that it is.
>
>Second, Dave Wilner did talk to me about this problem before he gave his
>talk.  I could not find my notes where I had detailed the description of the
>problem.  So, I winged it and I sure did get it wrong.  Sorry Dave.
>
>ACKNOWLEDGMENTS
>
>First, thanks to Mike for writing a very nice description of the talk.  I
>think I have had probably 400 people send me copies.  You gave me the push
>to write the part of the Mars Pathfinder End-of-Mission report that I had
>been procrastinating doing.
>
>Special thanks to Steve Stolper for helping me do this.  The biggest thanks
>should go to the software team that I had the privilege of leading and whose
>expertise allowed us to succeed: Pam Yoshioka, Dave Cummings, Don Meyer, 
>Karl Schneider, Greg Welz, Rick Achatz, Kim Gostelow, Dave Smyth, 
>Steve Stolper.   Also, Miguel San Martin, Sam Sirlin, Brian Lazara (WRS), 

>Mike Deliman (WRS), Lisa Stanley (WRS)
>
>Glenn Reeves, Mars Pathfinder Flight Software Cognizant Engineer
>glenn.e.reeves@jpl.nasa.gov