Messages in this thread | | | Subject | Re: [PATCH 0/3] cpu: idle state framework for offline CPUs. | From | Peter Zijlstra <> | Date | Mon, 17 Aug 2009 09:15:57 +0200 |
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On Mon, 2009-08-17 at 11:54 +0530, Dipankar Sarma wrote: > On Sun, Aug 16, 2009 at 11:53:22PM +0200, Peter Zijlstra wrote: > > On Mon, 2009-08-17 at 01:14 +0530, Balbir Singh wrote: > > > 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) > > > > I still don't get it... _why_ should this be exposed in the guest > > kernel? Why not let the hypervisor manage a guest's offline cpus in a > > way it sees fit? > > 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.
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.
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