RT-Preempt Patch使能

RT-Preempt Patch对Linux kernel的主要改造包括：

Making in-kernel locking-primitives (using spinlocks) preemptible though reimplementation with rtmutexes:
Critical sections protected by i.e. spinlock_t and rwlock_t are now preemptible. The creation of non-preemptible sections (in kernel) is still possible with raw_spinlock_t (same APIs like spinlock_t)
Implementing priority inheritance for in-kernel spinlocks and semaphores. For more information on priority inversion and priority inheritance please consultIntroduction to Priority Inversion
Converting interrupt handlers into preemptible kernel threads: The RT-Preempt patch treats soft interrupt handlers in kernel thread context, which is represented by a task_struct like a common userspace process. However it is also possible to register an IRQ in kernel context.
Converting the old Linux timer API into separate infrastructures for high resolution kernel timers plus one for timeouts, leading to userspace POSIX timers with high resolution.

1. What is "latency"?
------------------------------------------------------------------------------The term latency, when used in the context of the RT Kernel, is the
time interval between the occurance of an event and the time when that
event is "handled" (typically "handled" means running some thread as a
result of the event). Latencys that are of interest to kernel
programmers (and application programmers) are: - the time between when an interrupt occurs and the threadwaiting for that interrupt is run- the time between a timer expiration and the thread waiting forthat timer to run- The time between the receipt of a network packet and when thethread waiting for that packet runsYes, the timer and network example above are usually examples of the
more general interrupt case (most timers signal expiration with an
interrupt and most network interface cards signal packet arrival with
an interrupt as well), but the main idea is that an "event" occurs and
there is some elapsed time interval which concludes with the kernel
successfully handling the event.So, latency in and of itself is not a bad thing; there is always a
delay between occurance and completion of an event. What is bad is
when latency becomes excessive, meaning that the delay exceeds some
arbitrary threshold. What is this threshold? That's for each
application to define. A threshold or "deadline" is what defines a
real time application: meeting deadlines means success, missing
deadlines (exceeding the threshold) means failing to be real time.

https://rt.wiki.kernel.org/index.php/Cyclictest

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