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

[Julien Grall <julien.grall@arm.com>]

Hello Peng,

On 21/09/16 09:38, Peng Fan wrote:
> On Tue, Sep 20, 2016 at 01:17:04PM -0700, Stefano Stabellini wrote:
>> On Tue, 20 Sep 2016, Julien Grall wrote:
>>> On 20/09/2016 20:09, Stefano Stabellini wrote:
>>>> On Tue, 20 Sep 2016, Julien Grall wrote:
>>>>> On 20/09/2016 12:27, George Dunlap wrote:
>>>>>> On Tue, Sep 20, 2016 at 11:03 AM, Peng Fan <van.freenix@gmail.com>
>>>>>> wrote:
>>>>>>> On Tue, Sep 20, 2016 at 02:54:06AM +0200, Dario Faggioli wrote:
>>>>>>>> On Mon, 2016-09-19 at 17:01 -0700, Stefano Stabellini wrote:
>>>>>>>>> On Tue, 20 Sep 2016, Dario Faggioli wrote:
>>>> It is harder to figure out which one is supposed to be
>>>> big and which one LITTLE. Regardless, we could default to using the
>>>> first cluster (usually big), which is also the cluster of the boot cpu,
>>>> and utilize the second cluster only when the user demands it.
>>>
>>> Why do you think the boot CPU will usually be a big one? In the case of Juno
>>> platform it is configurable, and the boot CPU is a little core on r2 by
>>> default.
>>>
>>> In any case, what we care about is differentiate between two set of CPUs. I
>>> don't think Xen should care about migrating a guest vCPU between big and
>>> LITTLE cpus. So I am not sure why we would want to know that.
>>
>> No, it is not about migrating (at least yet). It is about giving useful
>> information to the user. It would be nice if the user had to choose
>> between "big" and "LITTLE" rather than "class 0x1" and "class 0x100", or
>> even "A7" or "A15".
>
> As Dario mentioned in previous email,
> for dom0 provide like this:
>
> dom0_vcpus_big = 4
> dom0_vcpus_little = 2
>
> to dom0.
>
> If these two no provided, we could let dom0 runs on big pcpus or big.little.
> Anyway this is not the important point for dom0 only big or big.little.
>
> For domU, provide "vcpus.big" and "vcpus.little" in xl configuration file.
> Such as:
>
> vcpus.big = 2
> vcpus.litle = 4
>
>
> According to George's comments,
> Then, I think we could use affinity to restrict little vcpus be scheduled on little vcpus,
> and restrict big vcpus on big vcpus. Seems no need to consider soft affinity, use hard
> affinity is to handle this.
>
> We may need to provide some interface to let xl can get the information such as
> big.little or smp. if it is big.little, which is big and which is little.
>
> For how to differentiate cpus, I am looking the linaro eas cpu topology code,
> The code has not been upstreamed (:, but merged into google android kernel.
> I only plan to take some necessary code, such as device tree parse and
> cpu topology build, because we only need to know the computing capacity of each pcpu.
>
> Some doc about eas piece, including dts node examples:
> https://git.linaro.org/arm/eas/kernel.git/blob/refs/heads/lsk-v4.4-eas-v5.2:/Documentation/devicetree/bindings/scheduler/sched-energy-costs.txt

I am reluctant to take any non-upstreamed bindings in Xen. There is a 
similar series going on the lklm [1].

But it sounds like it is a lot of works for little benefits (i.e giving 
a better name to the set of CPUs). The naming will also not fit if in 
the future hardware will have more than 2 kind of CPUs.

[...]

> I am not sure, but we may also need to handle mpidr for ARM, because big and little vcpus are supported.

I am not sure to understand what you mean here.

Regards,

[1] https://lwn.net/Articles/699569/

-- 
Julien Grall

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