Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

Jeremy Fitzhardinge
2007-03-07T00:24:16Z

Thomas Gleixner wrote:
> All paravirt users probably want to have NO_HZ, so PARAVIRT might simply
> depend on NO_HZ. Of course I might be wrong :)
>

Xen can deal either way, but tickless is certainly preferred.

> OTOH the stolen time accounting should be fixed in general and not rely
> on it happens to work now assumptions. And it should be done for _ALL_
> hypervisors in the same way, i.e. in the generic code.
>

Yep. We'll need to come up with a common story for that.

>> This is probably something the Xen folks will want
>> also, since I think Xen itself only gets 100hz hard timer, and so it can
>> implement at best a oneshot virtual timer with 100hz resolution. Any
>> objections to us doing something like this?
>>

Xen has a nanosecond resolution one-shot timer which I'm using for
this. There's also a 100Hz tick which gets in the way a bit (it will
appear as a stream of spurious timeouts), but we'll turn that off soon.

>> 3) clockevent set_next_event interface is suboptimal for paravirt (and
>> probably realtime-ish uses). The problem is that the expiry is passed
>> as a relative time. On paravirt, an arbitrary amount of (stolen) time
>> may have passed since the delta was computed and when the timer device
>> is programmed, causing that next interrupt to be too far out in the
>> future. It seems a better interface for set_next_event would be to pass
>> the current time and the absolute expiry. Actually, I sent email to
>> Thomas and Ingo about this (and some other clockevents/hrtimer feedback)
>> in July 2006, but never heard back. Thoughts?
>>
>
> There is no problem for realtime uses, as the reprogramming path is
> running with local interrupts disabled. I can see the point for paravirt
> and I'm not opposed to change / expand the interface for that. It might
> be done by an extra clockevents feature flag, which requests absolute
> time instead of relative time.
>

I'm not sure how much different it makes overall. It's true that
absolute time would be a more useful interface, but because the guest
vcpu can be preempted at any time, we could miss the timeout
regardless. In Xen if you set a timeout for the past you get an
immediate interrupt; I presume the clockevent code can deal with that?

J