linux-kernel - Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

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Date:	Thu, 30 Jun 2011 23:54:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

On Wed, 2011-06-29 at 11:29 -0400, Glauber Costa wrote:
> +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> +       if (static_branch((&paravirt_steal_rq_enabled))) {
> +               int is_idle;
> +               u64 st;
> +
> +               is_idle = ((rq->curr != rq->idle) ||
> +                               irq_count() != HARDIRQ_OFFSET);

Now that hurt my brain. If the vcpu is idle, why does it want to run?
How can an idle vcpu ever rack up steal time?

Also, what's that HARDIRQ_OFFSET bit about? sorely lacking in
explanation, and the Changelog to this patch is about as bad as the last
one.

> +               __touch_steal_time(is_idle, delta, &st);
> +
> +               steal = st * TICK_NSEC;
> +
> +               delta -= steal;
> +       }
> +#endif 
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