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

Paolo Bonzini
2017-02-24T15:35:09Z

On 24/02/2017 14:04, Marcelo Tosatti wrote:
>>>>> 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.
>
> 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).

There isn't, but this patchset is providing a direct path from a task to
cpufreq-userspace. This is as close as you can get to a per-task frequency.

> 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".

You could just pin them at the highest non-TurboBoost frequency until a
guest runs. That's assuming that they are idle and, because of
isol_cpus/nohz_full, they would be almost always in deep C state anyway.

Paolo