Re: [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes

[Rik van Riel <riel@redhat.com>]

Langsdorf, Mark wrote:
>> Here is the missing piece of the puzzle.   A platform
>> hypercall operation to get system wide idle time.
>>
>> I believe Mark's changes, together with this little
>> patch, are the way we can get cpufreq working on
>> Xen with the minimal amount of code duplication.
>>
>> Duplicating code anywhere, whether it be inside the
>> hypervisor or in some Xen-only userland package, will
>> only lead to bit rot and make Xen maintenance more
>> painful.
> 
> This code looks like it returns the amount of time
> spent in the running runstate, not the idle time.
> Am I completely missing something?

It is the time that the CPU's _idle domain_ has
spent in RUNSTATE_running.  This corresponds to
the idle time on each physical CPU.

> For reference, the ondemand governor calculates
> idle time as the sum of cpustat.idle and cpustat.iowait.
> I'd think the equivalent would be the sum of
> RUNSTATE_runnable and RUNSTATE_blocked.

The hypervisor has no concept of iowait on a
physical CPU basis.  It only tracks domain
VCPUs and the idle domain VCPUs.

Since the idle domain VCPUs never migrate
between CPUs, they reflect physical CPU idle
time.

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