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

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Date:   Fri, 24 Feb 2017 17:54:39 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls

On Friday, February 24, 2017 04:34:52 PM Paolo Bonzini wrote:
> 
> 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.

Good discussion so far, but it should be happening on the linux-pm list.

Would it be possible to repost the patches with a CC to linux-pm?

Thanks,
Rafael