Re: vtime accounting

Rik van Riel
2017-03-24T15:04:47Z

On Wed, 2017-03-15 at 09:43 +0100, Christoffer Dall wrote:

> Yes, I think the key is whether you end up taking a timer interrupt
> before or after switchign PF_VCPU.  So you can spend X jiffies in the
> guest, come back, change PF_VCPU (timer still hasen't expired), and
> then
> the timer expires immediately afterwards, and the whole block of
> jiffies
> that are incremented as a result of the timer gets accounted as
> kernel
> time.
>
> (Note that jiffies on clocksourced architectures is an arbitrary
> number,
> which somehow scales according to the clocksource frequency,
> depending
> on the CONFIG_HZ, so a single timer interrupt can increment jiffies
> by
> more than 1.
>

That really should not happen with nohz_full.

The housekeeping CPU should get a timer interrupt
every jiffy, unless something changed recently that
I am not aware of.

Frederic, am I totally off base?