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

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

Hello,

On 19/09/2016 10:36, Peng Fan wrote:
> On Mon, Sep 19, 2016 at 10:09:06AM +0200, Julien Grall wrote:
>> Hello Peng,
>>
>> On 19/09/2016 04:08, van.freenix@gmail.com wrote:
>>> From: Peng Fan <peng.fan@nxp.com>
>>>
>>> This patchset is to support XEN run on big.little SoC.
>>> The idea of the patch is from
>>> "https://lists.xenproject.org/archives/html/xen-devel/2016-05/msg00465.html"
>>>
>>> There are some changes to cpupool and add x86 stub functions to avoid build
>>> break. Sending The RFC patchset out is to request for comments to see whether
>>> this implementation is acceptable or not. Patchset have been tested based on
>>> xen-4.8 unstable on NXP i.MX8.
>>>
>>> I use Big/Little CPU and cpupool to explain the idea.
>>> A pool contains Big CPUs is called Big Pool.
>>> A pool contains Little CPUs is called Little Pool.
>>> If a pool does not contains any physical cpus, Little CPUs or Big CPUs
>>> can be added to the cpupool. But the cpupool can not contain both Little
>>> and Big CPUs. The CPUs in a cpupool must have the same cpu type(midr value for ARM).
>>> CPUs can not be added to the cpupool which contains cpus that have different cpu type.
>>> Little CPUs can not be moved to Big Pool if there are Big CPUs in Big Pool,
>>> and versa. Domain in Big Pool can not be migrated to Little Pool, and versa.
>>> When XEN tries to bringup all the CPUs, only add CPUs with the same cpu type(same midr value)
>>> into cpupool0.
>>
>> As mentioned in the mail you pointed above, this series is not enough to make
>> big.LITTLE working on then. Xen is always using the boot CPU to detect the
>> list of features. With big.LITTLE features may not be the same.
>>
>> And I would prefer to see Xen supporting big.LITTLE correctly before
>> beginning to think to expose big.LITTLE to the userspace (via cpupool)
>> automatically.
>
> Do you mean vcpus be scheduled between big and little cpus freely?

By supporting big.LITTLE correctly I meant Xen thinks that all the cores 
has the same set of features. So the feature detection is only done the 
boot CPU. See processor_setup for instance...

Moving vCPUs between big and little cores would be a hard task (cache 
line issue, and possibly feature) and I don't expect to ever cross this 
in Xen. However, I am expecting to see big.LITTLE exposed to the guest 
(i.e having big and little vCPUs).

>
> This patchset is to use cpupool to block the vcpu be scheduled between big and
> little cpus.
>
>>
>> See for instance v->arch.actlr = READ_SYSREG32(ACTLR_EL1).
>
> Thanks for this. I only expose cpuid to guest, missed actlr. I'll check
> the A53 and A72 TRM about AArch64 implementationd defined registers.
> This actlr can be added to the cpupool_arch_info as midr.
>
> Reading "vcpu_initialise", seems only MIDR and ACTLR needs to be handled.
> Please advise if I missed anything else.

Have you check the register emulation?

>
>>
>>>
>>> Thinking an SoC with 4 A53(cpu[0-3]) + 2 A72(cpu[4-5]), cpu0 is the first one
>>> that boots up. When XEN tries to bringup secondary CPUs, add cpu[0-3] to
>>> cpupool0 and leave cpu[4-5] not in any cpupool. Then when Dom0 boots up,
>>> `xl cpupool-list -c` will show cpu[0-3] in Pool-0.
>>>
>>> Then use the following script to create a new cpupool and add cpu[4-5] to
>>> the cpupool.
>>> #xl cpupool-create name=\"Pool-A72\" sched=\"credit2\"
>>> #xl cpupool-cpu-add Pool-A72 4
>>> #xl cpupool-cpu-add Pool-A72 5
>>> #xl create -d /root/xen/domu-test pool=\"Pool-A72\"
>>
>> I am a bit confused with these runes. It means that only the first kind of
>> CPUs have pool assigned. Why don't you directly create all the pools at boot
>> time?
>
> If need to create all the pools, need to decided how many pools need to be created.
> I thought about this, but I do not come out a good idea.
>
> The cpupool0 is defined in xen/common/cpupool.c, if need to create many pools,
> need to alloc cpupools dynamically when booting. I would not like to change a
> lot to common code.

Why? We should avoid to choose a specific design just because the common 
code does not allow you to do it without heavy change.

We never came across the big.LITTLE problem on x86, so it is normal to 
modify the code.

> The implementation in this patchset I think is an easy way to let Big and Little
> CPUs all run.

I care about having a design allowing an easy use of big.LITTLE on Xen. 
Your solution requires the administrator to know the underlying platform 
and create the pool.

In the solution I suggested, the pools would be created by Xen (and the 
info exposed to the userspace for the admin).

>
>>
>> Also, in which pool a domain will be created if none is specified?
>>
>>> Now `xl cpupool-list -c` shows:
>>> Name            CPU list
>>> Pool-0          0,1,2,3
>>> Pool-A72        4,5
>>>
>>> `xl cpupool-list` shows:
>>> Name               CPUs   Sched     Active   Domain count
>>> Pool-0               4    credit       y          1
>>> Pool-A72             2   credit2       y          1
>>>
>>> `xl cpupool-cpu-remove Pool-A72 4`, then `xl cpupool-cpu-add Pool-0 4`
>>> not success, because Pool-0 contains A53 CPUs, but CPU4 is an A72 CPU.
>>>
>>> `xl cpupool-migrate DomU Pool-0` will also fail, because DomU is created
>>> in Pool-A72 with A72 vcpu, while Pool-0 have A53 physical cpus.
>>>
>>> Patch 1/5:
>>> use "cpumask_weight(cpupool0->cpu_valid);" to replace "num_online_cpus()",
>>> because num_online_cpus() counts all the online CPUs, but now we only
>>> need Big or Little CPUs.
>>
>> So if I understand correctly, if the boot CPU is a little CPU, DOM0 will
>> always be able to only use little ones. Is that right?
>
> Yeah. Dom0 only use the little ones.

This is really bad, dom0 on normal case will have all the backends. It 
may not be possible to select the boot CPU, and therefore always get a 
little CPU.

Creating the pool at boot time would have avoid a such issue because, 
unless we expose big.LITTLE to dom0 (I would need the input of George 
and Dario for this bits), we could have a parameter to specify which set 
of CPUs (e.g pool) to allocate dom0 vCPUs.

Note, that I am not asking you to implement everything. But I think we 
need a coherent view of big.LITTLE support in Xen today to go forward.

Regards,

-- 
Julien Grall

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