Hi, all,
I am a newbie on RTAI. If I have two task, one is periodic task which has lower priority and the other one is aperiodic task which has higher priority, and the aperiodic task need more process time than periodic task, would the periodic task was blocked by aperiodic task?
If periodic task was blocked, so it is not periodic anymore because it can not get period time from cpu.

Any help is appreciate.

Yes, the lower prio task would be blocked by the higher prio task until the higher prio task is finished (assuming priority based scheduling with no resource sharing).

The scenario you describe is a missed deadline. The task is defined as a periodic task but it can't complete before the next period due to high CPU load. To fix this you must either optimize your schedule, tasks or scheduling algo. in some way to meet the timing requirements or upgrade your HW.

If the aperiodic task's deadline is not a hard one but the periodic task's deadline is, you may consider lowering aperiodic task's prio.