When SMP was first introduced to Linux back in 2.0 kernel days, it was done with the "big kernel lock". The Big Kernel Lock was used so that when one processor tried to enter the kernel it had to aquire this lock, and if another processor was already in the kernel it would block and wait for it to finish.

In the 2.2 series of kernels, this "Big Kernel Lock" was slowly removed from parts of the kernel. This allowed more than one processor to be executing kernel code simultaniously, but only for some operations, others hadn't been updated yet for one reason or another, so they aquired the Big Kernel Lock.

Now, in 2.4, the Big Kernel Lock has gone away, everything is protected by it's own little locks. this means the kernel is "Reentrant". Now, the Premptive Kernel Patch works by allowing another process on the same CPU preempt another process that is already in the kernel. This gives much better response, since if you have a high priority task thats blocked waiting on some data, and a low priority process running, as soon as the data is available for the High priority task it can resume running where as before this patch it would have to wait until the low priority task left the kernel.

Surprisingly enough, in most situations this actually improves performance! The reason is that processes that have been blocked get an oppertunity to run as soon as possible doing a bit of work and then usually blocking quickly afterwards allowing the machine to return to it's previous job.