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Re: [Xen-devel] Vcpu allocation for a newly created domU

  • To: Sandesh <sandesh.ahiremath@xxxxxxxxx>
  • From: "Grzegorz Miłoś" <gm281@xxxxxxxxx>
  • Date: Fri, 20 Jun 2008 10:41:41 +0100
  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 20 Jun 2008 02:42:03 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

>> When you enter the scheduler (and also __context_switch) you are
>> running VCPU=current on CPU=smp_processor_id().
> this is true only if current is not set to the next vcpu. but if you
> look at context_switch() just before calling __context_switch ,
> set_curent(next) is done. At this point current is actually the next
> vcpu to be scheduled.

First of all: I only said that current->proccessor ==
smp_processor_id() will hold on scheduler entry. But this equality is
actually stronger, I believe it will hold always.

>>  At this point in time
>> current->processor and smp_processor_id() will match.
> So vcpu->processor need not match smp_processor_id()....

Except that the next vcpu (or current, after set_current has been
executed) will also have ->processor filed set to smp_processor_id().
You have to explicitly migrate VCPUs between CPUs (and set their
->processor field in the process) to make them runnable on a different
CPU. For example the credit scheduler does that in
csched_runq_steal(). If it deems a VCPU to be migratable
(__csched_vcpu_is_migrateable) it will remove it from it's current
runqueue (__runq_remove) and update the processor field (vc->processor
= cpu)

Is that clear now?


> Thanks,
>  Sandesh
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