I think I solved the question: pthread_setaffinity_np is based on sched_setaffinity syscall. Actually sched_setaffinity performs a read_lock(&tasklist_lock); However, with the introduction of the PREEMPT_RT patch, the read_lock is preemptible, thats why the Thread T0 Yields in favor of T2. I think the sched.c should be revised regarding the PREEMPT_RT patch, and the scheduling related syscalls should adopt non pre-emptible (e.g. raw_spinlock_t) spinlocks rather than preemptible one, in order to avoid unwilling behaviors like the one showed. Regards, Primiano On Mon, Apr 19, 2010 at 11:45 AM, Primiano Tucci <p.tucci@xxxxxxxxx> wrote: > Hi all, > I am an Italian researcher and I am working on a Real Time scheduling > infrastructure. I am currently using Linux Kernel 2.6.29.6-rt24-smp > (PREEMPT-RT Patch) running on a Intel Q9550 CPU. > I am experiencing strange behaviors with the pthread_setaffinity_np API. > > This is my scenario, I have 4 Real Time Threads (SCHED_FIFO) > distributed as follows: > > T0 : CPU 0, Priority 2 (HIGH) > T1 : CPU 1, Priority 2 (HIGH) > T3 : CPU 0, Priority 1 (LOW) > T4 : CPU 1, Priority 1 (LOW) > > So T0 and T1 are actually the "big bosses" on CPUs #0 and #1, T3 and > T4, instead, never execute (let's assume that each thread is a simple > busy wait that never sleeps/yields) > Now, at a certain point, from T0 code, I want to migrate T4 from CPU > #1 to #0, keeping its low priority. > Therefore I perform a pthread_setaffinity_np from T0 changing T4 mask > from CPU #1 to #0. > > In this scenario it happens that T3 (that should never execute since > there is T0 with higher priority currently running on the same CPU #0) > "emerge" and executes for a bit. > It seems that the pthread_setaffinity_np syscall is somehow > "suspensive" for the time needed to migrate T4 and let the scheduler > to execute T3 for that bunch of time. > > Is this behavior expected (I did not find any documentation about > this)? How can avoid it? > > Thanks in advance, > Primiano > > -- > Primiano Tucci > http://www.primianotucci.com > -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
