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

[Julien Grall <julien.grall@xxxxxxx>]

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@xxxxxxxxx>
> > > > > > 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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