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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jun 09, 2010 at 05:16:47PM +0300, Avi Kivity wrote:
> On 06/09/2010 04:23 PM, Marcelo Tosatti wrote:
> >
> >>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 :(
> 
> Any idea where that came from?  Even looping on vcpus, that took 4
> usec per vcpu.

The rearming of timers (that emulate LAPIC) cause the host hrtimer
interrupt handler to re-execute its "run expired timers" loop. If timing
is unfortunate, it'll have to do that several times.