One-shot-oriented event timers management

[Gary Jennejohn]

On Mon, 30 Aug 2010 12:11:48 +0200
Gary Jennejohn <gljennjohn at googlemail.com> wrote:

> On Mon, 30 Aug 2010 13:07:38 +0300
> Alexander Motin <mav at FreeBSD.org> wrote:
> 
> > Gary Jennejohn wrote:
> > > Hmm.  I applied your patches and am now running the new kernel.  But I
> > > only installed the new kernel and didn't do make buildworld installworld.
> > > 
> > > Mu systat -vm 1 doesn't look anything like yours.  I'm seeing about 2300
> > > interrupts per second and most of those are coming from the hpet timers:
> > > 
> > > 1122 hpet0:t0
> > > 1124 hpet0:t1
> > 
> > It means 1000Hz of hardclock (hz) events mixed with 127Hz of statclock
> > (stathz) events. HPET timer here works in one-shot mode handling it.
> > 
> > > So, what else did you do to reduce interrupts so much?
> > > 
> > > Ah, I think I see it now.  My desktop has only C1 enabled.  Is that it?
> > > Unfortunately, it appears that only C1 is supported :(
> > 
> > Yes, as I have said, at this moment empty ticks skipped only while CPU
> > is in C2/C3 states. In C1 state there is no way to handle lost events on
> > wake up. While it may be not very dangerous, it is not very good.
> > 
> 
> Too bad.  I'd say that systems which are limited to C1 don't benefit
> much (or not at all) from your changes.
> 

OK, this is purely anecdotal, but I'll report it anyway.

I was running pretty much all day with the patched kernel and things
seemed to be working quite well.

Then, after about 7 hours, everything just stopped.

I had gkrellm running and noticed that it updated only when I moved the
mouse.

This behavior leads me to suspect that the timer interrupts had stopped
working and the mouse interrupts were causing processes to get scheduled.

Unfortunately, I wasn't able to get a dump and had to hit reset to
recover.

As I wrote above, this is only anecdotal, but I've never seen anything
like this before applying the patches.

--
Gary Jennejohn