Re: Another dumb question about Voluntary Kernel Preemption Patch

From: Lee Revell
Date: Fri Jul 23 2004 - 19:26:23 EST

On Wed, 2004-07-21 at 17:38, Timothy Miller wrote:
> Lee Revell wrote:

> > My
> > understanding is that the kernel is already preemptible anytime that a
> > spin lock (including the BKL) is not held, and that the voluntary kernel
> > preemption patch adds some scheduling points in places where it is safe
> > to sleep, but preemption is disabled because we are holding the BKL, and
> > that the number of these should approach zero as the kernel is improved
> > anyway.
> That's confusing to me. It was my understanding that the BKL is used to
> completely lock down the kernel so that no other CPU can have a process
> get into the kernel... something like how SMP was done under 2.0.

Yes, I was incorrect. The vountary kernel preemption patch takes
sections that are non-preemptible (aka holding a spinlock) and that
would otherwise run for an unbounded time and adds logic to break out of
those loops, releasing any locks, in order to allow a higher priority
process to run. It is voluntary because even though you are in a
non-preemptible section you voluntarily release any locks and yield to a
higher priority process. It has nothing to do with the BKL as such.


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