linux-kernel - Re: [accounting regression since rc1] scheduler updates

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Date:	Mon, 20 Aug 2007 21:12:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	heiko.carstens@...ibm.com, Paul Mackerras <paulus@...ba.org>
Subject: Re: [accounting regression since rc1]  scheduler updates


* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:

> > If a virtual CPU is idle then i think the "real = steal, virtual = 
> > 0" way of thinking about idle looks a bit unnatural to me - wouldnt 
> > it be better to think in terms of "steal = 0, virtual = real" ? 
> > Basically a virtual CPU can idle at "perfect speed", without the 
> > host "stealing" any cycles from it. And with that way of thinking, 
> > if s390 passed in the real-idle-time value to the new callbacks 
> > below it would all fall into place. Hm?
> 
> How you think about an idle cpu depends on your viewpoint. The source 
> for the virtual cpu time on s390 is the cpu timer. This timer is 
> stopped when a virtual cpu looses the physical cpu, so it seems 
> natural to me to think that real=steal, virtual=0 because the cpu 
> timer is stopped while the cpu is idle. The other way of thinking 
> about it is as valid though.

my thinking is this: the structure of "idle time" only matters if it can 
be observed from "within" a virtual machine - via timers. Are on s390 
any of the typical app-visible timers (timer_list, etc.) driven by the 
virtual tick? [which slows down if a virtual CPU is scheduled away by 
the host/monitor/hypervisor?] Or is the virtual tick only affecting 
scheduling/cpu-accounting statistics in essence?

	Ingo
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