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Re: [Xen-devel] [RFC 0/5] xen/arm: support big.little SoC

On Wed, 2016-09-21 at 10:22 +0100, George Dunlap wrote:
> On 21/09/16 09:38, Peng Fan wrote:
> > User may change the hard affinity of a vcpu, so we also need to
> > block a little
> > vcpu be scheduled to a big physical cpu. Add some checking code in
> > xen,
> > when chaning the hard affnity, check whether the cap of a vcpu is
> > compatible
> > with the cap of the physical cpus.
Yes, restricting affinity changes would indeed will be necessary. Note
that this is not a limit of the 'pinning based' implementation. Even
if/when we'll have in-scheduler support, trying to pin a LITTLE vcpu to
a big pcpu should, AFAIUI, will have to fail.

I was thinking to some parameter, that we can set from xl (applicable also on 
non-big.LITTLE or non-heterogeneous configurations), for askting Xen to make 
the hard-affinity 'immutable'. That would be rather simple to do.

But I like Peng's idea of validating hard-affinity against the class even 
better! :-)

> Dario, what do we do with vNUMA / soft affinity?
We do nothing actually. I mean, for now, we just accept whatever the
user asks, which might well be setting soft, or even hard, affinity of
all the domain to a set of nodes when the domain itself does not have
any memory.

This is actually another case where either immutability or restriction
of the changes that we allow to the (soft, in this case) affinity would
be useful. I'd always liked to introduce the logic that at least would
print a warning is something that looks really bad is being done, but
never got down to actually do that.

Maybe this will be the chance to improve wrt to this too! :-)

<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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