Re: [RFC][PATCH 1/3] Introduce a workqueue to deliver PIT timer interrupts.

Marcelo Tosatti
2010-06-09T13:50:16Z

On Tue, Jun 08, 2010 at 04:36:16PM -1000, Zachary Amsden wrote:
> >+ pt->timer.function = pit_timer_fn;
> >
>
> I am happy to see this. I thought kvm_timer_fn was a step
> backwards; it was too general of a function to justify the savings
> of 20 some odd lines of code.

This was not done to save 20 lines of code:

http://www.mail-archive.com/kvm@vger.kernel.org/msg18640.html

> Is there any chance that using a workqueue might help the problem of
> hrtimers firing too quickly? I wanted to return HR_NORESTART from
> pit_timer_fn always, then restart the hrtimer on delivery, but
> because of unreliable delivery, it wasn't clear how to do that.
>
> Perhaps the workqueue can be used to restart the timer instead,
> avoiding problems of impossibly small timeouts causing hrtimers to
> run amok.

It should be rearmed on ACK ideally. How come delivery is unreliable?

BTW, a massive amount of hrtimers (_single_ non-tickless 32-vcpu guest
with LAPIC) results in:

hrtimer: interrupt took 122448 ns

in the host. So don't even need impossibly small timeouts :(