Messages in this thread | | | Date | Wed, 16 Sep 2009 21:54:59 +0530 | From | Dipankar Sarma <> | Subject | Re: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework |
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On Wed, Sep 16, 2009 at 05:32:51PM +0200, Peter Zijlstra wrote: > On Wed, 2009-09-16 at 20:58 +0530, Dipankar Sarma wrote: > > On Tue, Sep 15, 2009 at 02:11:41PM +0200, Peter Zijlstra wrote: > > > On Tue, 2009-09-15 at 17:36 +0530, Gautham R Shenoy wrote: > > > > This patchset contains the offline state driver implemented for > > > > pSeries. For pSeries, we define three available_hotplug_states. They are: > > > > > > > > online: The processor is online. > > > > > > > > offline: This is the the default behaviour when the cpu is offlined > > > > > > > > inactive: This cedes the vCPU to the hypervisor with a cede latency > > > > > > > > Any feedback on the patchset will be immensely valuable. > > > > > > I still think its a layering violation... its the hypervisor manager > > > that should be bothered in what state an off-lined cpu is in. > > > > The problem is that all hypervisor managers cannot figure out what sort > > of latency guest OS can tolerate under the situation. They wouldn't know > > from what context guest OS has ceded the vcpu. It has to have > > some sort of hint, which is what the guest OS provides. > > I'm missing something here, hot-unplug is a slow path and should not > ever be latency critical..?
You aren't, I did :)
No, for this specific case, latency isn't an issue. The issue is - how do we cede unused vcpus to hypervisor for better energy management ? Yes, it can be done by a hypervisor manager telling the kernel to offline and make a bunch of vcpus "inactive". It does have to choose offline (release vcpu) vs. inactive (cede but guranteed if needed). The problem is that long ago we exported a lot of hotplug stuff to userspace through the sysfs interface and we cannot do something inside the kernel without keeping the sysfs stuff consistent. This seems like a sane way to do that without undoing all the virtual cpu hotplug infrastructure in different supporting archs.
Thanks Dipankar
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