kernel: return from interrupt

[Robert Watson]

On Thu, 11 Nov 2004, Anurekh Saxena wrote:

> I was under the impression that the 5.3 release had an option for full
> preemption.  If I am correct, why does the kernel refuse to schedule on
> a return_from_interrupt if its not going back to userland? I can
> understand this being a problem if interrupts were nested, or return
> from a page fault in a critical section. Please correct me if I am
> wrong, but if a *high* priority interrupt thread is ready to run, it
> should be given a chance. Presuming the *interrupted* kernel path is
> going to give up the CPU fast enough is probably not a good idea. 
> 
> I hope I have sent this to the right mailing list. 

Even normal "options PREEMPTION" should do this.  I know from tracing the
kernel in 6.x that that's the way the system behaves out of the box; with
PREEMPTION turned on in 5.x you should see the same behavior.  One thing I
often do see, FWIW, is that if you're on an SMP box, the ithread will get
scheduled to run immediately on another CPU that's idle, so you won't
actually preempt the thread on the current CPU other than for the
interrupt handler.  What behavior are you seeing that suggests this isn't
happening with PREEMPTION compiled in?

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Principal Research Scientist, McAfee Research