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

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Date:   Fri, 24 Feb 2017 10:18:59 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls



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