linux-kernel - Re: [PATCH v2 4/6] KVM-GST: KVM Steal time registration

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Date:	Tue, 01 Feb 2011 17:17:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, aliguori@...ibm.com,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: [PATCH v2 4/6] KVM-GST: KVM Steal time registration

On Tue, 2011-02-01 at 13:53 -0200, Glauber Costa wrote:
> 
> And since the granularity of the cpu accounting is too coarse, we end up
> with much more steal time than we should, because things that are less
> than 1 unity of cputime, are often rounded up to 1 unity of cputime.

See, that! is the problem, don't round up like that.

What you can do is: steal_ticks = steal_time_clock() / TICK_NSEC, or
simply keep a steal time delta and every time it overflows
cputime_one_jiffy insert a steal-time tick.

Venki might have created some infrastructure for doing this with the
IRQ_TIME accounting mess, but irqtime_account_process_tick() still gives
me a head-ache.



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