linux-kernel - Re: [PATCH 0/4] Alter steal-time reporting in the guest

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Message-ID: <20130307031124.GB2385@drongo>
Date:	Thu, 7 Mar 2013 14:11:24 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Michael Wolf <mjw@...ux.vnet.ibm.com>,
	Glauber Costa <glommer@...allels.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com, gleb@...hat.com,
	kvm@...r.kernel.org, peterz@...radead.org, mingo@...hat.com,
	anthony@...emonkey.ws
Subject: Re: [PATCH 0/4] Alter steal-time reporting in the guest

On Wed, Mar 06, 2013 at 09:52:16PM -0300, Marcelo Tosatti wrote:
> On Wed, Mar 06, 2013 at 10:29:12AM -0600, Michael Wolf wrote:
> > I looked at doing that once but was told that I was changing the
> > interface in an unacceptable way, because now I was not reporting all of
> > the elapsed time.  I agree it would make things simpler.
> 
> Pointer to that claim, please?

Back in about 2004 or 2005 or so I was looking at changing how user
and system times were calculated (in the context of trying to find a
better way to report resources used by a thread in an SMT processor).
I found that utilities such as top expected the deltas in the
/proc/stat numbers to add up to elapsed time, and would report strange
and inconsistent results if that wasn't the case.  Unfortunately at
this distance I don't recall the exact details.  I don't know whether
the expectation that the deltas in the /proc/stat numbers over a
period of time add up to the elapsed real time is documented anywhere,
but I wouldn't be at all surprised if some programs depend on it, so
it's better to maintain that property.

Paul.
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