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

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Date:   Fri, 24 Feb 2017 08:50:00 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 Fri, Feb 24, 2017 at 10:18:59AM +0100, Paolo Bonzini wrote:
> 
> 
> 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.

No, its per cpu.

> >> 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.

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

This is a special usecase where only the app in the guest knows 
whats the most appropriate frequency at a given time.
This is what cpufreq-userspace is supposed to allow userspace to do, 
but in this case "userspace" is the guest, so i don't 
see this as an abuse at all.

Timeshared setups are by definition not deterministic: 
your task A could be interrupted by another task B 
with results similar to a lower frequency being set.

So saying that:

"Our frequency scaling interface goes against the idea -- guest kernel
 cannot schedule multiple userspaces on the same vCPU, because they
 could
 conflict by overriding frequency."

Assumes that, in a timeshared system, an application is guaranteed a
particular frequency. But that does not make sense: its a timeshared
system in the first place, there is no determinism regarding execution
time.

Moreover, there is no notion of "per-task CPU frequency" in Linux
(there could be, this whole governor business with user
being responsible for setting up the governor is pretty sucky
IMO).