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Re: [Xen-devel] Core parking feature enable

Jan Beulich wrote:
>>>> On 29.02.12 at 13:41, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
>> Liu, Jinsong wrote:
>>> Jan Beulich wrote:
>>>>>>> On 17.02.12 at 18:48, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
>>>>>>> wrote:
>>>>> Jan Beulich wrote:
>>>>>>>>> On 17.02.12 at 09:54, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
>>>>>>>>> wrote:
>>>>>>> Core parking is a power control feature and it can co-work with
>>>>>>> NPTM to control system power budget through online/offline some
>>>>>>> CPUs in the system. These patches implement core parking feature
>>>>>>> for xen. They consist of 2 parts: dom0 patches and xen
>>>>>>> hypervisor patches. 
>>>>>>> At dom0 side, patches include
>>>>>>> [Patch 1/3] intercept native pad (Processor Aggregator Device)
>>>>>>> logic, providing a native interface for natvie platform and a
>>>>>>> paravirt template for paravirt platform, so that os can
>>>>>>> implicitly hook to proper ops accordingly; [Patch 2/3] redirect
>>>>>>> paravirt template to Xen pv ops; [Patch 3/3] implement Xen pad
>>>>>>> logic, and when getting pad device notification, it hypercalls
>>>>>>> to Xen hypervisor for core parking. Due to the characteristic
>>>>>>> of xen continue_hypercall_on_cpu, dom0 seperately send/get core
>>>>>>> parking request/result; 
>>>>>>> At Xen hypervisor side, patches include
>>>>>>> [Patch 1/2] implement hypercall through which dom0 send core
>>>>>>> parking request, and get core parking result;
>>>>>>> [Patch 2/2] implement Xen core parking. Different core parking
>>>>>>> sequence has different power/performance result, due to cpu
>>>>>>> socket/core/thread topology. This patch provide power-first and
>>>>>>> performance-first policies, users can choose core parking policy
>>>>>>> on their own demand, considering power and performance tradeoff.
>>>>>> Does this really need to be implemented in the hypervisor? All
>>>>>> this boils down to is a wrapper around cpu_down() and cpu_up(),
>>>>>> which have hypercall interfaces already. So I'd rather see this
>>>>>> as being an extension to Dom0's pCPU management patches (which
>>>>>> aren't upstream afaict)... 
>>>>>> Jan
>>>>> It's a design choice. Core parking is not only a wrapper around
>>>>> cpu_down/up, it also involves policy algorithms which depend on
>>>>> physical cpu topology and cpu_online/present_map, etc. Implement
>>>>> core parking at dom0 side need expose all those information to
>>>>> dom0, with potential issues (like coherence), while dom0 still
>>>>> need do same work as hypervisor. Our idea is to keep dom0 as ACPI
>>>>> parser, then hypercall and do rest things at hypervisor side.
>>>> Actually, after some more thought, I don't even think this ought to
>>>> be implemented in the Dom0 kernel, but in user space altogether.
>>>> Afaict all information necessary is already being exposed.
>>> No, user space lack necessary information. If I didn't
>>> misunderstand, it has some dom0-side dependencies not ready now,
>>> like 
>>> 1. sysfs interface, and exposing xen pcpu topology and maps;
>>> 2. intecept pad notify and call usermodehelper;
>>> 3. a daemon to monitor/policy core parking (daemon enable when linux
>>> run as pvops under xen (kernel acpi_pad disable now), daemon disable
>>> when linux run under baremetal (kernel acpi_pad enable now))
>>> Seems keep same approach as native kernel which handle acpi_pad in
>>> kernel side (for us, in hypervisor side) is a reasonable choice. Per
>>> my understanding core parking is a co-work part of NPTM, the whole
>>> process is basically a remote controller-microengine-bios-kernel
>>> process, not necessarily involve user action.
>> Any comments?
> No - I continue to disagree that this needs to be done outside of
> user space (the fact that certain necessary kernel pieces aren't in
> pv-ops is no excuse, nor is it that native does this in the kernel -
> that would at most allow for implementing it in the kernel, but still
> won't justify doing it in the hypervisor).

Jan, could you elaborate more your thoughts? like
- the pros of user space approach (and cons, if it has);
- the disadvantages of hypervisor approach;

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