One of the features of PREEMPT_RT is that it converts interrupt handlers to running in their own threads (with some exceptions, I believe), instead of being tacked on top of whatever thread context was active at the time like with the softirq approach the "normal" kernel uses. This allows the scheduler to better decide what should run (e.g. your RT process rather than serving interrupts for downloading cat pictures).
One of the features of PREEMPT_RT is that it converts interrupt handlers to running in their own threads (with some exceptions, I believe), instead of being tacked on top of whatever thread context was active at the time like with the softirq approach the "normal" kernel uses. This allows the scheduler to better decide what should run (e.g. your RT process rather than serving interrupts for downloading cat pictures).