On Mon, Aug 17, 2009 at 01:28:15PM +0530, Dipankar Sarma wrote: > On Mon, Aug 17, 2009 at 09:15:57AM +0200, Peter Zijlstra wrote: > > On Mon, 2009-08-17 at 11:54 +0530, Dipankar Sarma wrote: > > > For most parts, we do. The guest kernel doesn't manage the offline > > > CPU state. That is typically done by the hypervisor. However, offline > > > operation as defined now always result in a VM resize in some hypervisor > > > systems (like pseries) - it would be convenient to have a non-resize > > > offline operation which lets the guest cede the cpu to hypervisor > > > with the hint that the VM shouldn't be resized and the guest needs the > > > guarantee > > > to get the cpu back any time. The hypervisor can do whatever it wants > > > with the ceded CPU including putting it in a low power state, but > > > not change the physical cpu shares of the VM. The pseries hypervisor, > > > for example, clearly distinguishes between the two - "rtas-stop-self" call > > > to resize VM vs. H_CEDE hypercall with a hint. What I am suggesting > > > is that we allow this with an extension to existing interfaces because it > > > makes sense to allow sort of "hibernation" of the cpus without changing > > > any > > > configuration of the VMs. > > > > >From my POV the thing you call cede is the only sane thing to do for a > > guest. Let the hypervisor management interface deal with resizing guests > > if and when that's needed. > > That is more or less how it currently works - atleast for pseries hypervisor. > The current "offline" operation with "rtas-stop-self" call I mentioned > earlier is initiated by the hypervisor management interfaces/tool in > pseries system. This wakes up a guest system tool that echoes "1" > to the offline file resulting in the configuration change.
Should have said - echoes "0" to the online file. You don't necessarily need this in the guest Linux as long as there is a way for hypervisor tools to internally move Linux tasks/interrupts from a vcpu - async event handled by the kernel, for example. But I think it is too late for that - the interface has long been exported. > The OS involvement is necessary to evacuate tasks/interrupts > from the released CPU. We don't really want to initiate this from guests. > > > Thing is, you don't want a guest to be able to influence the amount of > > cpu shares attributed to it. You want that in explicit control of > > whomever manages the hypervisor. > > Agreed. But given a fixed cpu share by the hypervisor management tools, > we would like to be able to cede cpus to hypervisor leaving the hypervisor > configuration intact. This, we don't have at the moment and want to just > extend the current interface for this. > > Thanks > Dipankar > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
