Hi,
I’m having some trouble with the current thread priority mechanism.
Whenever I change my thread priority, it seems like it doesn’t work.
I’ve dig into this, and I think I’m hiting the classical thread priority inversion with mutex inheritance.
Here’s an example of what happen:
Thread H is running with high priority
Thread L is running with low priority
Thread M is running with medium priority
1) Thread L takes a mutex shared with Thread H.
2) Thread L is scheduled
3) Since thread H is highest priority, it's selected, and try to lock the mutex. It can't (L got it)
4) Thread M is then selected, and run. When it's done running, thread H is selected, but doesn't progress, and then Thread M is selected again (as so on)
What happens here, is that Thread M with medium priority is running more often than thread H with high priority. In a 3 threads system, thread M is running exclusively (said differently, thread H and L appear deadlocked during the whole life of thread M)
Under Posix system, this can be solved easily by giving priority inheritance to the mutex. So as soon as Thread H tries to take the mutex, the Thread L gets a instantaneous priority boosts to thread H priority. The situation unlocks itself.
Under windows, it’s done automatically by the scheduler, but on Linux, it’s not the case, you have to create mutex in a specific way for this to work:
pthread_mutex_t my_mutex;
pthread_mutexattr_t my_mutex_attr;
pthread_mutexattr_init(&my_mutex_attr);
pthread_mutexattr_setprotocol(&my_mutex_attr, PTHREAD_PRIO_INHERIT);
pthread_mutex_init(&my_mutex, &my_mutex_attr);
pthread_mutexattr_destroy(&my_mutex_attr);
// From here: http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.0/html/Realtime_Tuning_Guide/sect-Realtime_Tuning_Guide-Application_Tuning_and_Deployment-Mutex_options.html