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SubjectRe: [PATCH 0/3] cpu: idle state framework for offline CPUs.
* Dipankar Sarma <dipankar@in.ibm.com> [2009-08-16 23:56:29]:

> On Fri, Aug 14, 2009 at 01:30:21PM +0200, Pavel Machek wrote:
> > >
> > > It depends on the hypervisor implementation. On pseries (powerpc)
> > > hypervisor, for example, they are different. By offlining a vcpu
> > > (and in turn shutting a cpu), you will actually create a configuration
> > > change in the VM that is visible to other systems management tools
> > > which may not be what the system administrator wanted. Ideally,
> > > we would like to distinguish between these two states.
> > >
> > > Hope that suffices as an example.
> >
> > So... you have something like "physically pulling out hotplug cpu" on
> > powerpc.
>
> If any system can do physical unplug, then it should do "offline"
> with configuration changes reflected in the hypervisor and
> other system configuration software.
>
> > But maybe it is useful to take already offline cpus (from linux side),
> > and make that visible to hypervisor, too.
> >
> > So maybe something like "echo 1 > /sys/devices/system/cpu/cpu1/unplug"
> > would be more useful for hypervisor case?
>
> On pseries, we do an RTAS call ("stop-cpu") which effectively permantently
> de-allocates it from the VM hands over the control to hypervisor. The
> hypervisors may do whatever it wants including allocating it to
> another VM. Once gone, the original VM may not get it back depending
> on the situation.
>
> The point I am making is that we may not always want to *release*
> the CPU to hypervisor and induce a configuration change. That needs
> to be reflected by extending the existing user interface - hence
> the proposal for - /sys/devices/system/cpu/cpu<#>/state and
> /sys/devices/system/cpu/cpu<#>/available_states. It allows
> ceding to hypervisor without de-allocating. It is a minor
> extension of the existing interface keeping backwards compatibility
> and platforms can allow what make sense.
>


Agreed, I've tried to come with a little ASCII art to depict your
scenairos graphically


+--------+ don't need (offline)
| OS +----------->+------------+
+--+-----+ | hypervisor +-----> Reuse CPU
| | | for something
| | | else
| | | (visible to users)
| | | as resource changed
| +----------- +
V (needed, but can cede)
+------------+
| hypervisor | Don't reuse CPU
| | (CPU ceded)
| | give back to OS
+------------+ when needed.
(Not visible to
users as so resource
binding changed)


--
Balbir


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