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Re: [Xen-users] Setting CPU (core) frequency from dom0

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Subject: Re: [Xen-users] Setting CPU (core) frequency from dom0
From: Alberto Munoz <ajmunoz@xxxxxxxxxx>
Date: Thu, 20 Nov 2008 10:38:51 -0800
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Hi Kevin,

Thank you very much for the reply.

PLease see comments below.


On Wed, Nov 19, 2008 at 7:05 PM, Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
Your description is basically correct. Do you want to control cpu frequency from dom0 user level, or just try cpufreq governor in dom0?

I would like to control frequency from dom0. I want to arbitrarily set the  CPU frequency to a value and have it stay there (regardless of  the load on that CPU) until I change it.

For the former, we're currently adding more flexibility to xen cpufreq (such as user space governor) and control options to xenpm (such as manually changing freq), which is expected to reach xen upstream soon.

Yes, I thik this is what I need (ability to manually change frequency). Do you have an ETA (estimated time of arrival) for that functionality?
For the latter, it's suboptimal with limitations which is why xen cpufreq is develeoped. You may manually disable PROCESSOR_EXTERNAL_CONTROL in drivers/acpi/Kconfig to rebuild dom0 kernel for mode A.

OK.  I will do that.

By the way, I have been running onto an error  when booting (in mode A). The function  cpufreq_time_setup() in linux-2.6.18-xen.hg/arch/i386/kernel/time-xen.c is printing the following message during boot (and returning -ENODEV to its caller):

failed to set up cpufreq notifier

Further examination of the code shows that this happens when  notifier_chain_register() in linux-2.6.18-xen.hg/kernel/sys.c returns 0. The problem is that in the code I have, that function ALWAYS returns 0. The code for that function is as follows:

static int notifier_chain_register(struct notifier_block **nl,
        struct notifier_block *n)
    while ((*nl) != NULL) {
        if (n->priority > (*nl)->priority)
        nl = &((*nl)->next);
    n->next = *nl;
    rcu_assign_pointer(*nl, n);
    return 0;

think (but are not 100% sure yet) that as a result of this error, dom0 is not creating a cpufreq /sys interface (i.e

Do you happen to know what may be going on? perhaps I am just misreading the code? or perhaps not disabling PROCESSOR_EXTERNAL_CONTROL is what is causing this behavior? I will run some experiments on this last thing today.

Thanks again,



From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Alberto Munoz
Sent: Thursday, November 20, 2008 2:50 AM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Setting CPU (core) frequency from dom0

Hello everyone,

I am running some experiments that require changing the frequency of a CPU in a SunFire machine (32-core quad socket AMD Barcelona).

For what I have found (bits and pieces but not a comprehensive description), there are two meachisms for controlling core (most probably socket) frequency in Xen:

A- The older (xen-3.2.1) mechnism through dom0 (option cpufreq=dom0-kernel), which for what I gather uses a module in dom0 (cpufreqd?) that allows one to set cpu frequency.

B- The newer (xen-3.3), which has moved the governor to the hypervisor (option cpufreq=xen), which other than a utility to read processor P and C states (xenpm) does not (yet) allow user (dom0) control of CPU frequency.

My first question is whetherot what I have written above is accurate. My second question is where can I find detailed information on how to set up my system to make use of A above and be able to set cpu frequency "manually" from Dom0 (assuming that is still possible in Xen 3.3).

I am running the latest testing version of Xen 3.3 (about a week old).

Any information would be greatly appreciated.

Thank you,


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