Date: Thu, 11 Nov 2004 23:34:08 -0500 From: Anurekh Saxena <anurekh@gmail.com> To: Stephan Uphoff <ups@tree.com> Cc: freebsd-i386@freebsd.org Subject: Re: kernel: return from interrupt Message-ID: <aa26c8a90411112034794ff65f@mail.gmail.com> In-Reply-To: <1100213752.78635.32.camel@palm.tree.com> References: <aa26c8a904111109581723563d@mail.gmail.com> <1100213752.78635.32.camel@palm.tree.com>
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On Thu, 11 Nov 2004 17:55:52 -0500, Stephan Uphoff <ups@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 {
> /*
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