linux-kernel - Re: [PATCH 0/8] cpufreq: Auto-register with energy model

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Date:   Tue, 10 Aug 2021 10:35:21 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Andy Gross <agross@...nel.org>, Rafael Wysocki <rjw@...ysocki.net>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Cristian Marussi <cristian.marussi@....com>,
        Fabio Estevam <festevam@...il.com>,
        Kevin Hilman <khilman@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-omap@...r.kernel.org
Subject: Re: [PATCH 0/8] cpufreq: Auto-register with energy model



On 8/10/21 10:27 AM, Viresh Kumar wrote:
> On 10-08-21, 10:17, Lukasz Luba wrote:
>> Hi Viresh,
>>
>> I like the idea, only small comments here in the cover letter.
>>
>> On 8/10/21 8:36 AM, Viresh Kumar wrote:
>>> Provide a cpufreq driver flag so drivers can ask the cpufreq core to register
>>> with the EM core on their behalf. This allows us to get rid of duplicated code
>>> in the drivers and fix the unregistration part as well, which none of the
>>> drivers have done until now.
>>
>> The EM is never freed for CPUs by design. The unregister function was
>> introduced for devfreq devices.
> 
> I see. So if a cpufreq driver unregisters and registers again, it will
> be required to use the entries created by the registration itself,
> right ? Technically speaking, it is better to unregister and free any
> related resources and parse everything again.
> 
> Lets say, just for fun, I want to test two copies of a cpufreq driver

It's good that it's just for fun ;)

> (providing different set of freq-tables). I build both of them as
> modules, insert the first version, remove it, insert the second one.
> Ideally, this should just work as expected. But I don't think it will
> in this case as you never parse the EM stuff again.

The EM is directly used by scheduler in the hot-path, there are no
checks even if the EM if for CPUs. We are sure it's is for CPUs and
is always there for all CPUs.

I'm currently working on a EM v2 which would have stronger mechanisms
and do better job in this field. The patches are under internal review
and hopefully ready to post by the end of month.

> 
> Again, since the routine is there already, I think it is better/fine
> to just use it.

True, it doesn't harm, so I commented it in the patch 1/8 that it
could stay.

> 
>>> This would also make the registration with EM core to happen only after policy
>>> is fully initialized, and the EM core can do other stuff from in there, like
>>> marking frequencies as inefficient (WIP). Though this patchset is useful without
>>> that work being done and should be merged nevertheless.
>>>
>>> This doesn't update scmi cpufreq driver for now as it is a special case and need
>>> to be handled differently. Though we can make it work with this if required.
>>
>> The scmi cpufreq driver uses direct EM API, which provides flexibility
>> and should stay as is.
> 
> Right, so I left it as is for now.
>