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

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Date:	Tue, 06 Mar 2007 16:24:08 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	tglx@...utronix.de
CC:	Dan Hecht <dhecht@...are.com>, Zachary Amsden <zach@...are.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	ak@...e.de,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

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
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