Re: [PATCH v1 1/4] xen: add real time scheduler rt

[Dario Faggioli <dario.faggioli@citrix.com>]

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On mer, 2014-09-03 at 11:27 -0400, Meng Xu wrote:


> ​Actually it has three ways to implement this (if we go with
> batching):
> 1) As Dario mentioned, use period = 0 & budget=0 to indicate that this
> vcpu's parameters should not be changed;
> 2)​ We can first bounce out all VCPU's information, change the vcpus'
> information users specified, bounce the modified array back to
> hypervisor; and hypervisor set all vcpus' information based on the
> modified array. If a vcpu is not changed, its vcpu's information won't
> change, so its parameters won't change; 
>
That would mean issueing an hypercall for getting the array of vcpu
params, then changing the elements corresponding to the vcpu(s?) you
want to update, and issue the actual vcpus_set() hypercall.

Then, in Xen, you'll always change the parameters of _all_ the vcpus,
with some of the new values being exactly equal to the old ones.

This is viable, but quite unpleasant, especially the hypervisor part,
where you risk disturbing vcpus minding their own (possibly real-time)
business for no reason. If you go for it, you should at least avoid
this, by recognizing, inside Xen, that the parameters are the same and
not mess with the vcpu.

Even with that avoided, you still need a large array, as it is for 1)
and, this time, you bounce it up and down, so this, wrt 1), looks like a
loose-loose to me.

> 3) We only create arrays that have the vcpus to be modified. After we
> bound the array to hypervisor, hypervisor can use the vcpu's index to
> decide which vcpu's parameters should be modified.
>
This is an option. I like it less than 1), but I can see that, with 1),
changing the parameters of 1 vcpu of a 32 vcpus domain means pushing
down to Xen quite a bit of 0-s,. :-O

So, my preference is: "not 2". :-D

Regards,
Dario

-- 
<<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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