kernel: return from interrupt

[Anurekh Saxena]

On Thu, 11 Nov 2004 17:55:52 -0500, Stephan Uphoff <ups at tree.com> wrote:
> On Thu, 2004-11-11 at 12:58, Anurekh Saxena wrote:
> 
> 
> > Hi,
> >
> > 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.
> >
> > Thanks,
> > Anurekh
>  
> This should work if you have "options PREEMPTION" in your config file.
> You may also want to try "options FULL_PREEMPTION".

I wasnt looking at the  FULL_PREEMPTION option at all. With that
enabled, the kernel will
call mi_switch when it adds the thread to the runqueue. Thanks for the input.

> Can you describe your problems / observations?

I was expecting the common return_from_intr path to be used as a
preemption point.
It was an incorrect observation, and also probably wouldn't work with
the ast implementation.

> The exception seems to be fast interrupts.
> You may want to try the following untested patch to allow preemption
> triggered by fast interrupts.

That is interesting. I didn't see that the OWEPREEMPT flag is
deliberately cleared.
Do you why that is done? I dont see why a handler will explicitly call
maybe_preempt, but it
could try to add some thread to the runqueue.

Thanks for the feedback.

-Anurekh
> Index: intr_machdep.c
> ===================================================================
> RCS file: /cvsroot/src/sys/i386/i386/intr_machdep.c,v
> retrieving revision 1.11
> diff -u -r1.11 intr_machdep.c
> --- intr_machdep.c      3 Nov 2004 18:03:06 -0000       1.11
> +++ intr_machdep.c      11 Nov 2004 22:31:19 -0000
> @@ -205,7 +205,9 @@
>                 isrc->is_pic->pic_eoi_source(isrc);
>                 error = 0;
>                 /* XXX */
> +#if 0
>                 td->td_pflags &= ~TDP_OWEPREEMPT;
> +#endif
>                 critical_exit();
>         } else {
>                 /*