Re: [PATCH][RESUBMIT] don't schedule unplugged vcpus

[Ryan Harper <ryanh@us.ibm.com>]

* Keir Fraser <Keir.Fraser@cl.cam.ac.uk> [2005-06-08 16:57]:
> 
> On 8 Jun 2005, at 22:42, Ian Pratt wrote:
> 
> >I don't see why we care about vcpus that are down. From the user's 
> >point
> >of view they've gone for good -- it just happens that Xen hasn't freed
> >the memory in anticipation of it being used again. What do you think?
> >
> >I'd be inclined just to enter '-1' in the vcpu_to_cpu map. BTW: we 
> >could
> >make it an s16 rather than s32 at the same time. I think 32,768 CPUs
> >should keep be enough for anyone :-)
> 
> This is how I view it. We don't free the vcpu structure only because it 
> isn't reference counted. We can only be sure that noone has a reference 
> to the structure when the entire domain's refcnt falls to zero. Given 
> the small amount of memory involved, it's not worth the pain or 
> run-time cost of adding per-vcpu reference counts.
> 
> So VCPU_down == invisible outside Xen.

So, when I trigger a vcpu to go down via dom0 xm operation, I have to
trust that it worked?  I have no way of knowing at some point later
which vcpus are up or down?  I don't see any cost to this other than
during the getdominfo hcall.  

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253   T/L: 678-9253
ryanh@us.ibm.com