High resolution timers

Sleeping spinlocks

In a non-PREEMPT-RT preemption model spinlocks are mapped onto raw spinlocks. A task waiting for a spinlock held by another task spins until the task holding the spinlock releases it. Preemption is disabled in raw spinlock context. In PREEMPT_RT spinlocks are mapped onto sleeping spinlocks, and raw spinlocks retain their behavior. A task waiting for a sleeping spinlock goes to sleep and is woken up when the spinlock is released. In the sleeping spinlock context preemption is not disabled.

Threaded interrupt handler

The PREEMPT_RT patch forces the mechanism of threaded interrupt handlers. Due to this all interrupt handlers run in a threaded context except they are marked with the IRQF_NO_THREAD flag. This mechanism can be forced also in Linux mainline kernel without PREEMPT_RT patch by the kernel command line option threadirqs. But there is a small difference in the resulted behavior.

rt_mutex

All mutexes in the mainline Linux kernel are replaced by rt_mutexes. A rt_mutex implements priority inheritance to avoid priority inversion. This also applies to sleeping spinlocks and rwlocks. However, the holder of a semaphore can be preempted but does not participate in priority inheritance.

RCU

RCU mechanisms in mainline Linux are only preemptible if CONFIG_PREEMPT is set (Preemption model: “Low-Latency Desktop”). The PREEMPT_RT preemption models both use preemptible RCU mechanisms. Additionally the PREEMPT_RT patch eliminates RCU handling from all intermediate states and processes it only in its own thread.