Re: [Xen-users] Does Xen's Dom0 hypervisor based cpufreq support 'schedutil' governor?

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-08-23 13:52, lists@xxxxxxxxxxxx wrote:
> I use Xen 4.7.0_10 on Linux kernel 4.7.2
>
> I set/use Xen's Hypervisor based cpufreq, setting
>
>         cpufreq=xen
>
> Kernel 4.7.2 supports 'schedutil' governor. 
> (http://events.linuxfoundation.org/sites/events/files/slides/cpufreq_and_scheduler.pdf)
>
> From the xenpm manpage, you can set the governor to
>
>         set-scaling-governor  [cpuid] <gov> set scaling governor on CPU <cpuid> 
> or all as userspace/performance/powersave/ondemand
>
> but NOT 'schedutil'.
>
> Does Xen support using the kernel's 'schedutil'?  How's it configured?
I'm not 100% certain that I've ever completely understood cpufreq in 
Xen, but it's been my understanding that the xenpm set-scaling-governor 
command is used to set what governor Xen uses when cpufreq=xen is 
specified on the command line.  This does not use Linux's cpufreq 
governors or drivers at all.  To do that, you need to:
1. Set cpufreq=dom0 on the Xen command line.
2. Give Domain-0 a number of VCPU's exactly equal to the number of 
physical CPU's.
3. Pin Domain-0's VCPU's to PCPU's.
4. Possibly have some special support in Domain-0 (I'm not certain about 
this).
Assuming that I understand how this all works correctly, any Linux 
governor should work, but schedutil would make no sense since it 
interfaces with Linux's scheduler, not Xen's, which means it will be 
basing it's decisions on information that has nothing to do with actual 
load.  In general, there's very little practical use to having Domain-0 
set CPU frequency, as you can just as easily use xenpm from a separate 
daemon which monitors the relevant data to set the frequencies as you want.
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