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

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krcmar <rkrcmar@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls
Date: Fri, 24 Feb 2017 10:04:59 -0300	[thread overview]
Message-ID: <20170224130458.GA31610@amt.cnet> (raw)
In-Reply-To: <70575c62-b9c5-7100-193a-5f4b8bed258c@redhat.com>

On Fri, Feb 24, 2017 at 01:17:07PM +0100, Paolo Bonzini wrote:
> 
> 
> On 24/02/2017 12:50, Marcelo Tosatti wrote:
> >>>
> >>> 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.
> 
> Yeah, what I mean is that userspace program can be per-CPU, but it looks
> at all the processes running on that CPU ("the whole system").  This is
> very different from a guest, which is isolated.
> 
> >>>> 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.
> 
> 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.
> 
> Paolo

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).

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".