Re: [PATCH v2 2/2] [RFC] CPUFreq: Add support for cpu-perf-dependencies

[Nicola Mazzucato]

Hi Viresh, I'm glad it helped.

Please find below my reply.

On 10/9/20 6:39 AM, Viresh Kumar wrote:
> On 08-10-20, 17:00, Nicola Mazzucato wrote:
>> On 10/8/20 4:03 PM, Ionela Voinescu wrote:
>>> Hi Viresh,
>>>
>>> On Thursday 08 Oct 2020 at 16:32:41 (+0530), Viresh Kumar wrote:
>>>> On 07-10-20, 13:58, Nicola Mazzucato wrote:
>>>>> Hi Viresh,
>>>>>
>>>>> performance controls is what is exposed by the firmware through a protocol that
>>>>> is not capable of describing hardware (say SCMI). For example, the firmware can
>>>>> tell that the platform has N controls, but it can't say to which hardware they
>>>>> are "wired" to. This is done in dt, where, for example, we map these controls
>>>>> to cpus, gpus, etc.
>>>>>
>>>>> Let's focus on cpus.
>>>>>
>>>>> Normally we would have N of performance controls (what comes from f/w)
>>>>> that that correspond to hardware clock/dvfs domains.
>>>>>
>>>>> However, some firmware implementations might benefit from having finer
>>>>> grained information about the performance requirements (e.g.
>>>>> per-CPU) and therefore choose to present M performance controls to the
>>>>> OS. DT would be adjusted accordingly to "wire" these controls to cpus
>>>>> or set of cpus.
>>>>> In this scenario, the f/w will make aggregation decisions based on the
>>>>> requests it receives on these M controls.
>>>>>
>>>>> Here we would have M cpufreq policies which do not necessarily reflect the
>>>>> underlying clock domains, thus some s/w components will underperform
>>>>> (EAS and thermal, for example).
>>>>>
>>>>> A real example would be a platform in which the firmware describes the system
>>>>> having M per-cpu control, and the cpufreq subsystem will have M policies while
>>>>> in fact these cpus are "performance-dependent" each other (e.g. are in the same
>>>>> clock domain).
>>>>
>>>> If the CPUs are in the same clock domain, they must be part of the
>>>> same cpufreq policy.
>>>
>>> But cpufreq does not currently support HW_ALL (I'm using the ACPI
>>> coordination type to describe the generic scenario of using hardware
>>> aggregation and coordination when establishing the clock rate of CPUs).
>>>
>>> Adding support for HW_ALL* will involve either bypassing some
>>> assumptions around cpufreq policies or making core cpufreq changes.
>>>
>>> In the way I see it, support for HW_ALL involves either:
>>>
>>>  - (a) Creating per-cpu policies in order to allow each of the CPUs to
>>>    send their own frequency request to the hardware which will do
>>>    aggregation and clock rate decision at the level of the clock
>>>    domain. The PSD domains (ACPI) and the new DT binding will tell
>>>    which CPUs are actually in the same clock domain for whomever is
>>>    interested, despite those CPUs not being in the same policy.
>>>    This requires the extra mask that Nicola introduced.
>>>
>>>  - (b) Making deep changes to cpufreq (core/governors/drivers) to allow:
>>>    - Governors to stop aggregating (usually max) the information
>>>      for each of the CPUs in the policy and convey to the core
>>>      information for each CPU.
>>>    - Cpufreq core to be able to receive and pass this information
>>>      down to the drivers.
>>>    - Drivers to be able to have some per cpu structures to hold
>>>      frequency control (let's say SCP fast channel addresses) for
>>>      each of the CPUs in the policy. Or have these structures in the
>>>      cpufreq core/policy, to avoid code duplication in drivers.
>>>
>>> Therefore (a) is the least invasive but we'll be bypassing the rule
>>> above. But to make that rule stick we'll have to make invasive cpufreq
>>> changes (b).
>>
>> Regarding the 'rule' above of one cpufreq policy per clock domain, I would like
>> to share my understanding on it. Perhaps it's a good opportunity to shed some light.
>>
>> Looking back in the history of CPUFreq, related_cpus was originally designed
>> to hold the map of cpus within the same clock. Later on, the meaning of this
>> cpumask changed [1].
>> This led to the introduction of a new cpumask 'freqdomain_cpus'
>> within acpi-cpufreq to keep the knowledge of hardware clock domains for
>> sysfs consumers since related_cpus was not suitable anymore for this.
>> Further on, this cpumask was assigned to online+offline cpus within the same clk
>> domain when sw coordination is in use [2].
>>
>> My interpretation is that there is no guarantee that related_cpus holds the
>> 'real' hardware clock implementation. As a consequence, it is not true anymore
>> that cpus that are in the same clock domain will be part of the same
>> policy.
>>
>> This guided me to think it would be better to have a cpumask which always holds
>> the real hw clock domains in the policy.
>>
>>>
>>> This is my current understanding and I'm leaning towards (a). What do
>>> you think?
>>>
>>> *in not so many words, this is what these patches are trying to propose,
>>> while also making sure it's supported for both ACPI and DT.
>>>
>>> BTW, thank you for your effort in making sense of this!
>>>
>>> Regards,
>>> Ionela.
>>>
>>
>> This could be a platform where per-cpu and perf-dependencies will be used:
>>
>> CPU:              0    1    2    3    4    5    6    7
>> Type:             A    A    A    A    B    B    B    B
>> Cluster:         [                                    ]
>> perf-controls:   [  ] [  ] [  ] [ ]  [ ]  [ ]  [ ]  [ ]
>> perf-dependency: [                ]  [                ]
>> HW clock:        [                ]  [                ]
>>
>> The firmware will present 8 controls to the OS and each control is mapped to a
>> cpu device via the standard dt. This is done so we can achieve hw coordination.
>> What is required in these systems is to present to OS the information of which
>> cpus belong to which clock domain. In other words, when hw coordinates we don't
>> have any way at present in dt to understand how these cpus are dependent
>> each other, from performance perspective (as opposed to ACPI where we have
>> _PSD). Hence my proposal for the new cpu-perf-dependencies.
>> This is regardless whether we decide to go for either a policy per-cpu or a
>> policy per-domain.
>>
>> Hope it helps.
> 
> Oh yes, I get it now. Finally. Thanks for helping me out :)
> 
> So if I can say all this stuff in simple terms, this is what it will
> be like:
> 
> - We don't want software aggregation of frequencies and so we need to
>   have per-cpu policies even when they share their clock lines.
> 
> - But we still need a way for other frameworks to know which CPUs
>   share the clock lines (that's what the perf-dependency is all about,
>   right ?).
> 
> - We can't get it from SCMI, but need a DT based solution.
> 
> - Currently for the cpufreq-case we relied for this on the way OPP
>   tables for the CPUs were described. i.e. the opp-table is marked as
>   "shared" and multiple CPUs point to it.
> 
> - I wonder if we can keep using that instead of creating new bindings
>   for exact same stuff ? Though the difference here would be that the
>   OPP may not have any other entries.

I thought about it and looked for other platforms' DT to see if can reuse
existing opp information. Unfortunately I don't think it is optimal. The reason
being that, because cpus have the same opp table it does not necessarily mean
that they share a clock wire. It just tells us that they have the same
capabilities (literally just tells us they have the same V/f op points).
Unless I am missing something?

When comparing with ACPI/_PSD it becomes more intuitive that there is no
equivalent way to reveal "perf-dependencies" in DT.

Thank you for time on this.

Regards
Nicola

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