linux-kernel - Re: [PATCH 5/7] KVM-GST: KVM Steal time accounting

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Date:	Tue, 14 Jun 2011 12:10:03 +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 5/7] KVM-GST: KVM Steal time accounting

On Mon, 2011-06-13 at 19:31 -0400, Glauber Costa wrote:
> +static inline int touch_steal_time(int is_idle)
> +{
> +       u64 steal, st = 0;
> +
> +       if (static_branch(&paravirt_steal_enabled)) {
> +
> +               steal = paravirt_steal_clock(smp_processor_id());
> +
> +               steal -= this_rq()->prev_steal_time;
> +               this_rq()->prev_steal_time += steal;

If you move this addition below this test:

> +               if (is_idle || (steal < TICK_NSEC))
> +                       return 0;

that is, right here, then you don't loose tiny steal deltas and
subsequent ticks accumulate their steal time until you really
have a full steal tick to account.

I guess you want something different for the idle case though.

> +               while (steal > TICK_NSEC) {

			/* really, if we wanted a division we'd have written one */
			asm("" : "+rm" (steal));

> +                       steal -= TICK_NSEC;
> +                       st++;
> +               }
> +
> +               account_steal_time(st);
> +               return 1;
> +       }
> +       return 0;
> +} 


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