Re: [PATCH 2/5] xen: credit2: never consider CPUs outside of our cpupool.

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

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[Adding Anshul which also had to deal with something similar to this
 recently]

On Thu, 2017-01-19 at 09:08 +0100, Juergen Gross wrote:
> On 17/01/17 18:26, Dario Faggioli wrote:
> > Finally, given the wide number of issues similar to this that I've
> > found and
> > fixed in the last release cycle, I think it would be good to take a
> > stab at
> > whether the interface between cpupools and the schedulers could not
> > be
> > simplified. :-O
> 
> The first patch version for support of cpupools I sent used an own
> scheduler instance for the free cpus. Keir merged this instance with
> the one for Pool-0.
> 
Really? Because I more than once thought that something like that would
be a good idea. I even started to implement it once... I then dropped
it, because I was able to deal with that problem in another way, and
had more pressing priorities, but I always liked the concept.

> I think it might be a good idea to check whether some of the problems
> are going away with the free cpu scheduler idea again. 
>
I think some of them will.

AFAICT, out of the top of my head, that alone won't avoid having to
modify when and how a couple of scheduling hooks are called (the pCPU
data allocation and initialization ones), but most likely will simplify
making that change.

> Maybe it would
> even be a good idea to add a "null-scheduler" for this purpose
> supporting only one vcpu per pcpu. This scheduler could be used for
> high performance domains, too.
> 
Oh, I guess it can. At first, I was actually planning for such
scheduler to always and only select the appropriate idle vCPU... But it
can be extended to do something different, if wanted.

Let's see if others want to chime in. If not, I'll dust off the half-
backed patches I have for this and see if I can make them work.

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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