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Old 01-23-2009, 05:27 PM   #1
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Question pthread scheduling in ARM

I have a rather complex and interesting problem here, maybe somebody has run across this before:
I have an application running on an embedded ARM (in the chip we build). I've written a test application that will setup 2 threads (with priority) along with moving the "main" thread to a different priority (> 0). This test application runs just fine, compiled on the board or using cross tools. The Linux kernel is a 2.4.20 flavor.
This same logic is then moved into a library (.so) and I have the "real" application linking this .so. When this "real" application runs, when I get to the point of spawning the first thread (an ISR thread), the thread gets created/spawned, but I never return to the main thread to continue. This is not desired. This first thread down just does a sleep() and then increments a counter and prints it out. It will do this forever (until ^C).
I did find some issues with the pthreads implementation, it was throwing me some errors on some calls that I don't get on 3 other platforms (they are PPC based, same vintage Linux Kernel) and they all work just fine.
We've removed the customer driver's we had to build so as to rule out any interference from them. I get the same results.
For some reason, when running the "real" application, the thread is spawned but does not want to return to the main thread. The Code used is identical to what is in my test code that works.
Today, we rebuilt the .so libraries as .a's and created a static application, since the test code is just built as a stand-alone application, only links to -lrt, -lpthread. In this case, we get the same results, the thread is spawned, but never returns to the main thread.
This is very frustrating and is also critical: if a solution cannot be found, then the project is dead. The use of threading and thread priorities is mandatory for our application.
Has anyone experienced something similar with pthreads on ARM???
Any pointers would be greatly appreciated
Old 01-24-2009, 06:33 AM   #2
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Uhm only some pointer: first, not all operating systems are preemptible, so in very rare cases thread switching happens only because of I/O blocking calls (a thread waits for I/O, so other threads can continue). You can try with the pthread sched_yield that "causes the calling thread to relinquish the CPU".
Other thing, the "sleep" system call causes the whole process, not just a thread, to sleep. So, remember this before using it. Use "alarms" as a possible replacement for sleep in thread.
Old 02-04-2009, 12:09 AM   #3
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What is the scheduling algorithm and what is the scheduling priority for the spawned threads? If you are using Priority based scheduling and priority of the spawned threads is higher than the main, then you may be able to see such kind of behavior...

Old 02-05-2009, 04:05 PM   #4
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The question is which pthread implementation you use. The question is especially important in embedded environment, where you can have quite non-standard things. There are some that implementations that do not use system scheduling at all - they do it internally on system calls and pthread_yield(). Try to force the running thread to schedule and see what happens.


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