Re: vtime accounting

Paolo Bonzini
2017-03-14T17:09:48Z

On 14/03/2017 17:58, Radim Krčmář wrote:
>> I assume there's a good reason why we call guest_enter() and
>> guest_exit() in the hot path on every KVM architecture?
> I consider myself biased when it comes to jiffies, so no judgement. :)
>
> From what I see, the mode switch is used only for statistics.

vtime is only for statistics, but guest_enter/exit are important because
they enter an RCU extended quiescent state. This means that (physical)
CPUs running a guest are effectively "off" from the point of view of the
RCU accounting machinery. Not having to perform any RCU work is very
good for jitter.

Paolo