RE: [Xen-devel] Re: [PATCH 1/4] CPU online/offline support in Xen

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

On Friday, September 12, 2008 12:53 AM, Keir Fraser wrote:
> On 11/9/08 17:00, "Shan, Haitao" <haitao.shan@xxxxxxxxx> wrote:
> 
>> Hi, Keir,
>> 
>> Concerning the last running vcpu on the dying cpu, I have some
thought.
>> Yes, there would be a short time after the stop_machine_run when this
vcpu
>> v->processor == dying_cpu. But anyhow, we set fie __VPF_migrating
flag for
>> that vcpu and issued a schedule_softirq on the dying cpu.
>> This softirq should run immediately after stop_machine context, am I
right?
>> If so, by the time the schedule softirq is executed, this last vcpu
is
>> migrated away from this dying cpu. But saving of its context will be
delayed
>> to play_dead->sync_lazy_context. If another cpu issues the schedule
request
>> to this dying cpu
(vcpu_sleep_nosync->cpu_raise_softirq(vc->processor....))
>> during this time, the request will be serviced by the above code
sequence.
>> So it is safe in such cases. Am I missing something important? I am
not
>> quite confident on the statements, though.
> 
> I agree it looks safe.
> 
> By the way, have you considered using this hotplug functionality for
power
> management? If instead of for(;;) halt(); we instead hooked into Cx
> management and tried to get into as deep sleep as possible (possibly
even
> supporting the really deep sleeps that power off a whole socket and
mean you
> *have* to come back via real mode) then this would give a nice
> coarse-time-scale power management mechanism controllable from dom0.

Yes, that's one good suggestion and we can add deep sleep for offline
path.

> 
> I consider this might be a nice win for possibly less effort than is
being
> expended in trying to make idle residency times (and hence Cx
residency
> times) as long as possible.
> 

These two don't conflict. Cpu online/offline can't be used in small
interval due
to long latency and added overhead to whole system, but it makes sense 
when administrator realizes low cpu utilization in a relatively long
period like
in hrs. Current idle governor instead runs in fine-grained level to fit
the otherwise
cases.

Thanks,
Kevin


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