Re: [patch 0/3] KVM CPU frequency change hypercalls

Marcelo Tosatti
2017-02-24T13:07:25Z

On Fri, Feb 24, 2017 at 01:17:07PM +0100, Paolo Bonzini wrote:
>
>
> On 24/02/2017 12:50, Marcelo Tosatti wrote:
> >>>
> >>> On all other cpufreq implementations, these boundaries still need to
> >>> be set. Then, a "governor" must be selected. Such a "governor" decides
> >>> what speed the processor shall run within the boundaries. One such
> >>> "governor" is the "userspace" governor. This one allows the user - or
> >>> a yet-to-implement userspace program - to decide what specific speed
> >>> the processor shall run at.
> >> The userspace program sets a policy for the whole system.
> > No, its per cpu.
>
> Yeah, what I mean is that userspace program can be per-CPU, but it looks
> at all the processes running on that CPU ("the whole system"). This is
> very different from a guest, which is isolated.
>
> >>>> That's bad. This feature is broken by design unless it does proper
> >>>> save/restore across preemption.
> >>> Whats the current usecase, or forseeable future usecase, for save/restore
> >>> across preemption again? (which would validate the broken by design
> >>> claim).
> >> Stop a guest that is using cpufreq, start a guest that is not using it.
> >> The second guest's performance now depends on the state that the first
> >> guest left in cpufreq.
> >
> > Nothing forbids the host to implement switching with the
> > current hypercall interface: all you need is a scheduler
> > hook.
>
> Can it be done in vcpu_load/vcpu_put? But you still would have two
> components (KVM and sysfs) potentially fighting over the frequency, and
> that's still a bit ugly.
>
> Paolo

Change the frequency at vcpu_load/vcpu_put? Yes: call into
cpufreq-userspace. But there is no notion of "per-task frequency" on the
Linux kernel (which was the starting point of this subthread).

But if you configure all CPUs in the system as cpufreq-userspace,
then some other (userspace program) has to decide the frequency
for the other CPUs.

Which agent would do that and why? Thats why i initially said "whats the
usecase".