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

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Date:   Thu, 02 Feb 2017 15:47:55 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: [patch 0/3] KVM CPU frequency change hypercalls

Implement KVM hypercalls for the guest
to issue frequency changes.

Current situation with DPDK and frequency changes is as follows:
An algorithm in the guest decides when to increase/decrease
frequency based on the queue length of the device.

On the host, a power manager daemon is used to listen for
frequency change requests (on another core) and issue these
requests.

However frequency changes are performance sensitive events because:
On a change from low load condition to max load condition,
the frequency should be raised as soon as possible.
Sending a virtio-serial notification to another pCPU,
waiting for that pCPU to initiate an IPI to the requestor pCPU
to change frequency, is slower and more cache costly than
a direct hypercall to host to switch the frequency.

If the pCPU where the power manager daemon is running
is not busy spinning on requests from the isolated DPDK vcpus,
there is also the cost of HLT wakeup for that pCPU.

Moreover, the daemon serves multiple VMs, meaning that
the scheme is subject to additional delays from
queueing of power change requests from VMs.

A direct hypercall from userspace is the fastest most direct
method for the guest to change frequency and does not suffer
from the issues above.

The usage scenario for this hypercalls is for pinned vCPUs <-> pCPUs.