Force Win32 thread scheduling to a defined sequence based on priority

Question

I am an embedded programmer attempting to simulate a real time preemptive scheduler in a Win32 environment using Visual Studio 2010 and MingW (as two separate build environments). I am very green on the Win32 scheduling environment and have hit a brick wall with what I am trying to do. I am not trying to achieve real time behaviour - just to get the simulated tasks to run in the same order and sequence as they would on the real target hardware.

The real time scheduler being simulated has a simple objective - always execute the highest priority task (thread) that is able to run. As soon a task becomes able to run - it must preempt the currently running task if it has a priority higher than the currently running task. A task can become able to run due to an external event it was waiting for, or a time out/block time/sleep time expiring - with a tick interrupt generating the time base.

In addition to this preemptive behaviour, a task can yield or volunteer to give up its time slice because is is executing a sleep or wait type function.

I am simulating this by creating a low priority Win32 thread for each task that is created by the real time scheduler being simulated (the thread effectively does the context switching the scheduler would do on a real embedded target), a medium priority Win32 thread as a pseudo interrupt handler (handles simulated tick interrupts and yield requests that are signalled to it using a Win32 event object), and a higher priority Win32 thread to simulate the peripheral that generates the tick interrupts.

When the pseudo interrupt handler establishes that a task switch should occur it suspends the currently executing thread using SuspendThread() and resumes the thread that executes the newly selected task using ResumeThread(). Of the many tasks and their associated Win32 threads that may be created, only one thread that manages the task will ever be out of the suspended state at any one time.

It is important that a suspended thread suspends immediately that SuspendThread() is called, and that the pseudo interrupt handling thread executes as soon as the event telling it that an interrupt is pending is signalled - but this is not the behaviour I am seeing.

As an example problem that I already have a work around for: When a task/thread yields the yield event is latched in a variable and the interrupt handling thread is signalled as there is a pseudo interrupt (the yield) that needs processing. Now in a real time system as I am used to programming I would expect the interrupt handling thread to execute immediately that it is signalled because it has a higher priority than the thread that signals it. What I am seeing in the Win32 environment is that the thread that signals the higher priority thread continues for some time before being suspended - either because it takes some time before the signalled higher priority thread starts to execute or because it takes some time for the suspended task to actually stop running - I'm not sure which. In any case this can easily be correct by making the signally Win32 thread block on a semaphore after signalling the Win32 interrupt handling thread, and have the interrupt handling Win32 thread unblock the thread when it has finished its function (handshake). Effectively using thread synchronisation to force the scheduling pattern to what I need. I am using SignalObjectAndWait() for this purpose.

Using this technique the simulation works perfectly when the real time scheduler being simulated is functioning in co-operative mode - but not (as is needed) in preemptive mode.

The problem with preemptive task switching is I guess the same, the task continues to execute for some time after it has been told to suspend before it actually stops running so the system cannot be guaranteed to be left in a consistent state when the thread that runs the task suspends. In the preemptive case though, because the task does not know when it is going to happen, the same technique of using a semaphore to prevent the Win32 thead continuing until it is next resumed cannot be used.

Has anybody made it this far down this post - sorry for its length!

My questions then are:

• How I can force Win32 (XP) scheduling to start and stop tasks immediately that the suspend and resume thread functions are called - or - how can I force a higher priority Win32 thread to start executing immediately that it is able to do so (the object it is blocked on is signalled). Effectively forcing Win32 to reschedule its running processes.

• Is there some way of asynchronously stopping a task to wait for an event when its not in the task/threads sequential execution path.

• The simulator works well in a Linux environment where POSIX signals are used to effectively interrupt threads - is there an equivalent in Win32?

Thanks to anybody who has taken the time to read this long post, and especially thanks in advance to anybody that can hold my 'real time engineers' hand through this Win32 maze.

Answer 1

[EDIT] - simpler than the hack below - it seems just ensuring all the threads run on the same CPU core also fixes the problem :o) After all that. The only problem then is the CPU runs at nearly 100% and I'm not sure if the heat is damaging to it. [/EDIT]

Ahaa! I think I have a work around for this - but its ugly. The uglyness is kept in the port layer though.

What I do now is store the thread ID each time a thread is created to run a task (a Win32 thread is created for each real time task that is created). I then added the function below - which is called using trace macros. The trace macros can be defined to do whatever you want, and have proven very useful in this case. The comments in the code below explain. The simulation is not perfect, and all this does is correct the thread scheduling when it has already deviated from the real time scheduling whereas I would prefer it not to go wrong in the first place, but the positioning of the trace macros makes the code containing this solution pass all the tests:

void vPortCheckCorrectThreadIsRunning( void )
{
xThreadState *pxThreadState;

    /* When switching threads, Windows does not always seem to run the selected
    thread immediately.  This function can be called to check if the thread
    that is currently running is the thread that is responsible for executing
    the task selected by the real time scheduler.  The demo project for the Win32
    port calls this function from the trace macros which are seeded throughout 
    the real time kernel code at points where something significant occurs.
    Adding this functionality allows all the standard tests to pass, but users
    should still be aware that extra calls to this function could be required
    if their application requires absolute fixes and predictable sequencing (as
    the port tests do).  This is still a simulation - not the real thing! */
    if( xTaskGetSchedulerState() != taskSCHEDULER_NOT_STARTED )
    {
        /* Obtain the real time task to Win32 mapping state information. */
        pxThreadState = ( xThreadState * ) *( ( unsigned long * ) pxCurrentTCB );

        if( GetCurrentThreadId() != pxThreadState->ulThreadId )
        {
            SwitchToThread();
        }
    }
}

Answer 2

@Anthony W - thanks for the advice. I was running the Win32 threads that simulated the real time tasks at THREAD_PRIORITY_ABOVE_NORMAL, and the threads that ran the pseudo interrupt handler and the tick interrupt generator at THREAD_PRIORITY_HIGHEST. The threads that were suspended I was changing to THREAD_PRIORITY_IDLE in case that made any difference. I just tried your suggestion of using THREAD_PRIORITY_TIME_CRITICAL but unfortunately it didn't make any difference.

With regards to your question am I sure that the suspend and resume not happening immediately is the problem - well no I'm not. It is my best guess in an environment I am unfamiliar with. My thinking regarding the failure of suspend and resume to work immediately stems from my observation when a task yields. If I make the call to yield (signal [using a Win32 event] a higher priority Win32 thread to switch to the next real time task) I can place a break point after the yield and that gets hit before a break point in the higher priority thread. It is unclear whether a delay in signalling the event and the higher priority task running, or a delay in suspending the thread and the thread actually stopping running was causing this - but the behaviour was definitely observed. This was fixed using a semaphore handshake, but that cannot be done for preemptions caused by tick interrupts.

I know the simulation is not running as I expect because a set of tests that check the sequence of scheduling of real time tasks is failing. It is always possible the scheduler has a problem, or the test has a problem, but the test will run for weeks without failing on a real real time target so I'm inclined to think the test and the scheduler are ok. A big difference is on the real time target the tick frequency is 1 ms, whereas on the Win32 simulated target it is 15ms with quite a lot of variation even then.

@Remy - I have done quite a bit of reading about fibers today, and my conclusion is that for simulating the scheduler in cooperative mode they would be perfect. However, as far as I can see they can only be scheduled by the fibers themselves calling the SwitchToFiber() function. Can a thread be made to block on a timer or sleep so it runs periodically, effectively preempting the fiber that was running at the time? From what I have read the answer is no because blocking one fiber will block all fibers running in the thread. If it could be made to work, could the periodically executing fiber then call the SwitchToFiber() function to select the next fiber to run before again sleeping for a fixed period? Again I think the answer is no because once it switches to another fiber it will no longer be executing and so will not actually call the Sleep() function until the next time the executing fiber switches back to it. Please correct my logic here if I have got the wrong idea of how fibers work.

I think it could work if the periodic functionality could remain in its own thread, separate from the thread that executed the fibers - but (again from what I have read) I don't think a one thread can influence the execution of fibers running in a different thread. Again I would be grateful if you could correct my conclusions here if they are wrong.

Answer 3

Firstly, what priority values are you using for your threads?

If you set the high priority thread to THREAD_PRIORITY_TIME_CRITICAL it should run pretty much immediately --- only those threads associated with a real-time process will have higher priority.

Secondly, how do you know that the suspend and resume aren't happening immediately? Are you sure this is the problem?

You cannot force a thread to wait on something from outside without suspending the thread to inject the wait code; if SuspendThread isn't working for you then this isn't going to help.

The closest to a signal is probably QueueUserAPC, which will schedule a callback to run the next time the thread enters an "alertable wait state", e.g. by calling SleepEx or WaitForSingleObjectEx or similar.

Answer 4

If you need to do your own scheduling, then you might consider using fibers instead of threads. Fibers are like threads, in that they are separate blocks of executable code, however fibers can be scheduled in user code whereas threads are scheduled by the OS only. A single thread can host and manage scheduling of multiple fibers, and fibers can even schedule each other.