Is the default Raspbian shipped with the real time patch?

I'm using the `.config` of the default Raspbian installation as a base for a custom kernel compilation. In the section:

```
 >Kernel Features      
  -> Preemption Model (<choice> [=y])

```

The selected option is `Preemptible Kernel (Low-Latency Desktop)`.

The description of the option is:

> 
> This option reduces the latency of the kernel by making all kernel
>  code (that is not executing in a critical section) preemptible. This
>  allows reaction to interactive events by permitting a low priority
>  process to be preempted involuntarily even if it is in kernel mode
>  executing a system call and would otherwise not be about to reach a
>  natural preemption point. This allows applications to run more
>  'smoothly' even when the system is under load, at the cost of
>  slightly lower throughput and a slight runtime overhead to kernel
>  code. 
> 
> 
> Select this if you are building a kernel for a desktop or embedded
>  system with latency requirements in the milliseconds range.
> 
> 
> 

However, when I did some testing, in which I generated a software PWM on the GPIO with the frequency 1kHz, the behaviour was far from real time.

Does kernel preemption mean that rt\_patch is included in the kernel?
What else would I gain with the real time patch if the kernel is already preemptive? If the kernel is preemptive, why was the PWM unclean?