linux-kernel - Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation

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Date:	Mon, 31 Jan 2011 12:07:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>, tglx <tglx@...utronix.de>
Subject: Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation

On Fri, 2011-01-28 at 14:52 -0500, Glauber Costa wrote:
> +               u64 to = (get_kernel_ns() - vcpu->arch.this_time_out);
> +               /*
> +                * using nanoseconds introduces noise, which accumulates easily
> +                * leading to big steal time values. We want, however, to keep the
> +                * interface nanosecond-based for future-proofness.
> +                */
> +               to /= NSEC_PER_USEC;
> +               to *= NSEC_PER_USEC; 

This just doesn't make any sense at all, you use the most expensive and
accurate kernel interface to get ns resolution timestamps (ktime_get),
and then you truncate the stuff to usec for some totally weird and
unexplained reason.

If ktime_get is wrong all our time-keeping is out the window.



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