Re: [PATCH 0/4] CPUFreq statistics retrieved by drivers

[Lukasz Luba <lukasz.luba@arm.com>]

On 7/30/20 10:10 AM, Sudeep Holla wrote:
> On Thu, Jul 30, 2020 at 02:23:33PM +0530, Viresh Kumar wrote:
>> On 29-07-20, 16:12, Lukasz Luba wrote:
>>> The existing CPUFreq framework does not tracks the statistics when the
>>> 'fast switch' is used or when firmware changes the frequency independently
>>> due to e.g. thermal reasons. However, the firmware might track the frequency
>>> changes and expose this to the kernel.
>>>
>>> This patch set aims to introduce CPUfreq statistics gathered by firmware
>>> and retrieved by CPUFreq driver. It would require a new API functions
>>> in the CPUFreq, which allows to poke drivers to get these stats.
>>>
>>> The needed CPUFreq infrastructure is in patch 1/4, patch 2/4 extends
>>> ARM SCMI protocol layer, patches 3/4, 4/4  modify ARM SCMI CPUFreq driver.
>>
>> Are you doing this for the fast switch case or because your platform
>> actually runs at frequencies which may be different from what cpufreq
>> core has requested ?
>>
> 
> I think so.

For both cases, but fast switch is major and present. Thermal is not
currently implemented in SCP FW, but might be in future.

> 
>> I am also not sure what these tables should represent, what the
>> cpufreq core has decided for the CPUs or the frequencies we actually
>> run at, as these two can be very different for example if the hardware
>> runs at frequencies which don't match exactly to what is there in the
>> freq table. I believe these are rather to show what cpufreq and its
>> governors are doing with the CPUs.
>>
> 
> Exactly, I raised similar point in internal discussion and asked Lukasz
> to take up the same on the list. I assume it was always what cpufreq
> requested rather than what was delivered. So will we break the userspace
> ABI if we change that is the main question.

Thank you for confirmation. If that is the mechanism for tracking what
cpufreq governors are doing with the CPUs, then is clashes with
presented data in FW memory, because firmware is the governor.

> 
>> Over that I would like the userspace stats to work exactly as the way
>> they work right now, i.e. capture all transitions from one freq to
>> other, not just time-in-state. Also resetting of the stats from
>> userspace for example. All allocation and printing of the data must be
>> done from stats core, the only thing which the driver would do at the
>> end is updating the stats structure and nothing more. Instead of
>> reading all stats from the firmware, it will be much easier if you can
>> just get the information from the firmware whenever there is a
>> frequency switch and then we can update the stats the way it is done
>> right now. And that would be simple.
>>
> 
> Good point, but notifications may not be lightweight. If that is no good,
> alternatively, I suggested to keep these firmware stats in a separate
> debugfs. Thoughts ?

I agree that notifications might not be lightweight. Furthermore I think
this still clashes with the assumption that cpufreq governor decisions
are tracked in these statistics, not the firmware decision.

In this case I think we would have to create debugfs.
Sudeep do you think these debugfs should be exposed from the protocol
layer:
drivers/firmware/arm_scmi/perf.c
or maybe from the cpufreq scmi driver? I would probably be safer to have
it in the cpufreq driver because we have scmi_handle there.

Regards,
Lukasz