Re: [PATCH] clean up FIXME in do_timer_interrupt

Lee Revell
2005-03-04T20:52:01Z

On Fri, 2005-03-04 at 02:28 -0800, George Anzinger wrote:
> Lee Revell wrote:
> > On Thu, 2005-03-03 at 16:45 -0800, Andrew Morton wrote:
> >
> >>If efi_enabled is true and efi_set_rtc_mmss(xtime.tv_sec) returns zero, the
> >>new code will run set_rtc_mmss(xtime.tv_sec) whereas the old code won't.
> >
> >
> > Argh, I should know better then to send patches before having coffee.
> >
> > Here's a new patch. Still ugly, but might be a worthwhile cleanup.
>
> Lets ask the obvious question: Why isn't this update hung on a timer? It seems
> silly to check this 6000 times per update. I am sure we can sync a timer to the
> same degree we do timer interrupts, so there _must_ be some other reason. Right?
>

Thanks George, I knew there was an obvious question here, I just didn't
know what it was ;-).

The thin that brought this code to my attention is that with PREEMPT_RT
this happens to be the longest non-preemptible code path in the kernel.
On my 1.3 Ghz machine set_rtc_mmss takes about 50 usecs, combined with
the rest of timer irq we end up disabling preemption for about 90 usecs.
Unfortunately I don't have the trace anymore.

Anyway the upshot is if we hung this off a timer it looks like we would
improve the worst case latency with PREEMPT_RT by almost 50%. Unless
there is some reason it has to be done synchronously of course.

Lee