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

Dan Hecht
2007-03-07T00:35:51Z

On 03/06/2007 04:24 PM, Jeremy Fitzhardinge wrote:
> Thomas Gleixner wrote:
>>> 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?
>

That's the problem though, you won't know to set it for the past since
the expiry is relative. When the vcpu starts running again, it will set
the timer to expire X ns from now, not Xns from when the timer was
requested.