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

From	Paolo Bonzini <pbonzini@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [patch 0/3] KVM CPU frequency change hypercalls
Date	2017-02-24 10:20 +0100
Message-ID	<tekcF-6wz-13@gated-at.bofh.it> (permalink)
References	(1 earlier) <t6PdE-4aP-33@gated-at.bofh.it> <t6QMp-5eA-15@gated-at.bofh.it> <t6Rp8-5PP-25@gated-at.bofh.it> <te5GF-4ww-13@gated-at.bofh.it> <tecIa-11o-19@gated-at.bofh.it>
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On 24/02/2017 00:19, Marcelo Tosatti wrote:
>>> i.e. our feature implies userspace tasks pinned to isolated vCPUs.
> This is how cpufreq-userspace works:
> 
> 2.2 Governor
> ------------
> 
> 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.

>> 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.

I think this is abusing the userspace governor.  Unfortunately cpufreq
governors cannot be stacked.

Paolo

Thread

Re: [patch 0/3] KVM CPU frequency change hypercalls Paolo Bonzini <pbonzini@redhat.com> - 2017-02-23 18:50 +0100
  Re: [patch 0/3] KVM CPU frequency change hypercalls Marcelo Tosatti <mtosatti@redhat.com> - 2017-02-24 02:20 +0100
    Re: [patch 0/3] KVM CPU frequency change hypercalls Paolo Bonzini <pbonzini@redhat.com> - 2017-02-24 10:20 +0100
      Re: [patch 0/3] KVM CPU frequency change hypercalls Marcelo Tosatti <mtosatti@redhat.com> - 2017-02-24 13:00 +0100
        Re: [patch 0/3] KVM CPU frequency change hypercalls Paolo Bonzini <pbonzini@redhat.com> - 2017-02-24 13:20 +0100
          Re: [patch 0/3] KVM CPU frequency change hypercalls Marcelo Tosatti <mtosatti@redhat.com> - 2017-02-24 14:10 +0100
            Re: [patch 0/3] KVM CPU frequency change hypercalls Paolo Bonzini <pbonzini@redhat.com> - 2017-02-24 16:40 +0100
              Re: [patch 0/3] KVM CPU frequency change hypercalls "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-02-24 18:10 +0100
              Re: [patch 0/3] KVM CPU frequency change hypercalls Marcelo Tosatti <mtosatti@redhat.com> - 2017-02-28 22:20 +0100

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