* Dipankar Sarma <dipan...@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 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev