Re: [PATCH 0/3] cpu: idle state framework for offline CPUs.

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* 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