Re: [Qemu-devel] Rethinking missed tick catchup

[Anthony Liguori <anthony@codemonkey.ws>]

Gleb Natapov <gleb@redhat.com> writes:

> On Thu, Sep 13, 2012 at 10:56:56AM -0500, Anthony Liguori wrote:
>> Gleb Natapov <gleb@redhat.com> writes:
>> 
>> > On Thu, Sep 13, 2012 at 09:35:18AM -0500, Anthony Liguori wrote:
>> >> Gleb Natapov <gleb@redhat.com> writes:
>> >> 
>> >> > On Thu, Sep 13, 2012 at 09:06:29AM -0500, Anthony Liguori wrote:
>> >> >> "Daniel P. Berrange" <berrange@redhat.com> writes:
>> >> >> 
>> >> >> I think it's better for QEMU to talk to qemu-ga.  We can tell when a large
>> >> >> period of time has passed in QEMU because we'll accumulate a large
>> >> >> number of missed ticks.
>> >> >> 
>> >> > With RTC configured to use vm clock we will not.
>> >> 
>> >> Not for host suspend.  For stop and live migration, we stop vm_clock.
>> >> But QEMU isn't aware of host suspend so vm_clock cannot be stopped.
>> >> 
>> > Hmm, true. What about hooking into suspend and doing vmstop during
>> > suspend. 
>> 
>> Is suspend the only foreseeable way for this problem to happen?  I don't
>> think it is which is what concerns me about any approach that relies on
>> "hooking suspend".
>> 
> With RTC using real time clock setting host time far ahead of what is it
> will trigger same behaviour I think.
>
>> Also, I don't think there is a generic way to "hook suspend".
>> 
>> >> >> This could happen because of stop, host suspend, live migration to a
>> >> >> file, etc.
>> >> >> 
>> >> >> It's much easier for us to call into qemu-ga to do the time correction
>> >> >> whenever this event occurs than to try and have libvirt figure out when
>> >> >> it's necessary.
>> >> > And if guest does not have qemu-ga what is better inject interrupts like
>> >> > crazy for next 2 minutes or leave guest with incorrect time?
>> >> 
>> >> Yes, at least that's fixable by the end-user.  QEMU consuming 100% CPU
>> >> for a prolonged period of time isn't fixable.
>> >> 
>> > You mean yes to "leave guest with incorrect time"? QEMU will still
>> > consume 100% of cpu for some time calling qemu_timer callback millions
>> > times. timedrift code is not the right level to fix that.
>> 
>> Not if we put a cap on how many interrupts we'll try to catch up.
>> 
> Interrupts ctachup happens at another level. If guest was stopped for
> 24 hours while RTC was configured to 1kHz qemu_timer will fire callback
> 88473600 times. Each invocation will try to inject interrupt and fail
> incrementing coalesced_irq instead. You can cap coalesced_irq but
> callback will still fire 88473600 times.

That's a bug.

The next period calculation should not be based on the last period +
length of period but rather on the current time + delta to next period
boundary.

IOW, if we shouldn't arm timers to expire backwards in time from when
the event occurred.  That should be accounted as a missed tick.

Regards,

Anthony Liguori

>
>> As I mentioned previously, if we acrue more than X number of missed
>> ticks, we should simply declare bankruptcy and reset the counter.
>> 
>> When that occurs, *if* qemu-ga is present, we should ask qemu-ga to
>> reset the guest's clock based on reading the hardware clock via a
>> 'guest-resync-time' command.
>> 
>> If it isn't, time will be off.  Hopefully the guest is running NTP and
>> can correct itself.  Otherwise, at least the admin can manually fix the
>> time.
>> 
>> Regards,
>> 
>> Anthony Liguori
>> 
>> >
>> > --
>> > 			Gleb.
>
> --
> 			Gleb.