On Wed, Jun 09, 2010 at 11:17:41AM -1000, Zachary Amsden wrote: > On 06/09/2010 03:23 AM, Marcelo Tosatti wrote: > >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@xxxxxxxxxxxxxxx/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? > > Due to the various ways IRQ can be latched or not with different > routing. If you program a very short timeout, it should fire even > if the guest is slow to respond: > > i..i..i..i..i..i..i > ..a...a...a...a > > if you don't restart right away, only on ack, you can risk the > refire of the interrupt getting lost, especially if the guest > happens to reprogram the PIT while this is happening... > > Of course, it can be tracked properly, but it requires quite a few > more state variables and some careful reasoning to make sure it > doesn't drop irqs. > > This patch series looks as if it will make that sort of reasoning > easier to do :) > > >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 :( > 32-vcpu on how many pcpu? 8 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
