Re: [RFC PATCH 0/2] Boot time cpupools

[Juergen Gross <jgross@xxxxxxxx>]

On 17.11.21 11:26, Julien Grall wrote:
> Hi Luca,
>
> On 17/11/2021 09:57, Luca Fancellu wrote:
> > Currently Xen creates a default cpupool0 that contains all the cpu 
> > brought up
> > during boot and it assumes that the platform has only one kind of CPU.
> > This assumption does not hold on big.LITTLE platform, but putting 
> > different
> > type of CPU in the same cpupool can result in instability and security 
> > issues
> > for the domains running on the pool.
> I agree that you can't move a LITTLE vCPU to a big pCPU. However...
>
> > For this reason this serie introduces an architecture specific way to 
> > create
> > different cpupool at boot time, this is particularly useful on ARM 
> > big.LITTLE
> > platform where there might be the need to have different cpupools for 
> > each type
> > of core, but also systems using NUMA can have different cpu pool for 
> > each node.
> ... from my understanding, all the vCPUs of a domain have to be in the 
> same cpupool. So with this approach it is not possible:
>     1) to have a mix of LITTLE and big vCPUs in the domain
>     2) to create a domain spanning across two NUMA nodes
>
> So I think we need to make sure that any solutions we go through will 
> not prevent us to implement those setups.
> I can see two options here:
>    1) Allowing a domain vCPUs to be on a different cpupool
>    2) Introducing CPU class (see [1])
>
> I can't remember why Dario suggested 2) rather than 1) in the past. 
> @Dario, do you remember it?
Having vcpus of a domain in different cpupools would probably require a
major scheduling rework due to several accounting information being per
cpupool today with some of that data being additionally per domain. Not
to mention the per-domain scheduling parameters, which would need to
cope with different schedulers suddenly (imagine one cpupool using
credit2 and the other rt).

I'd rather use implicit pinning e.g. via a cpu class.


Juergen

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