linux-kernel - Re: [PATCH v2 08/17] PM: EM: Introduce runtime modifiable table

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Message-ID: <bcf297db-4878-84e2-842d-178b79d7abbe@arm.com>
Date:   Mon, 3 Jul 2023 16:58:14 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     rui.zhang@...el.com, amit.kucheria@...durent.com,
        amit.kachhap@...il.com, daniel.lezcano@...aro.org,
        viresh.kumar@...aro.org, len.brown@...el.com, pavel@....cz,
        Pierre.Gondois@....com, ionela.voinescu@....com,
        rostedt@...dmis.org, mhiramat@...nel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        rafael@...nel.org
Subject: Re: [PATCH v2 08/17] PM: EM: Introduce runtime modifiable table



On 5/30/23 11:18, Dietmar Eggemann wrote:
> On 12/05/2023 11:57, Lukasz Luba wrote:
>> This patch introduces the new feature: modifiable EM perf_state table.
>> The new runtime table would be populated with a new power data to better
>> reflect the actual power. The power can vary over time e.g. due to the
>> SoC temperature change. Higher temperature can increase power values.
>> For longer running scenarios, such as game or camera, when also other
>> devices are used (e.g. GPU, ISP) the CPU power can change. The new
>> EM framework is able to addresses this issue and change the data
>> at runtime safely. The runtime modifiable EM data is used by the Energy
>> Aware Scheduler (EAS) for the task placement.
> 
> It's important to say that EAS is the _only_user of the `runtime
> modifiable EM`. All the other users (thermal, etc.) are still using the
> default (basic) EM. IMHO, this fact drove the design here.

OK, I'll add that information in the header.

> 
>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>> ---
>>   include/linux/energy_model.h | 13 +++++++++++++
>>   kernel/power/energy_model.c  | 24 ++++++++++++++++++++++++
>>   2 files changed, 37 insertions(+)
>>
>> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
>> index cc2bf607191e..a616006a8130 100644
>> --- a/include/linux/energy_model.h
>> +++ b/include/linux/energy_model.h
>> @@ -36,9 +36,21 @@ struct em_perf_state {
>>    */
>>   #define EM_PERF_STATE_INEFFICIENT BIT(0)
>>   
>> +/**
>> + * struct em_perf_table - Performance states table, which can be
>> + *		runtime modifiable and protected with RCU
> 
> which is `runtime modifiable` ? So `runtime modifiable performance state
> table`? RCU is obvious since we have `struct rcu_head rcu`.

Thanks, 'Runtime modifiable performance state table' sounds better.

> 
>> + * @state:	List of performance states, in ascending order
>> + * @rcu:	RCU used for safe access and destruction
>> + */
>> +struct em_perf_table {
>> +	struct em_perf_state *state;
>> +	struct rcu_head rcu;
>> +};
>> +
>>   /**
>>    * struct em_perf_domain - Performance domain
>>    * @table:		List of performance states, in ascending order
>> + * @runtime_table:	Pointer to the runtime modified em_perf_table
> 
> s/modified/modifiable
> 
> [...]
> 
>> @@ -237,12 +238,23 @@ static int em_create_pd(struct device *dev, int nr_states,
>>   			return -ENOMEM;
>>   	}
>>   
>> +	runtime_table = kzalloc(sizeof(*runtime_table), GFP_KERNEL);
>> +	if (!runtime_table) {
>> +		kfree(pd);
>> +		return -ENOMEM;
>> +	}
>> +
>>   	ret = em_create_perf_table(dev, pd, nr_states, cb, flags);
>>   	if (ret) {
>>   		kfree(pd);
>> +		kfree(runtime_table);
>>   		return ret;
>>   	}
>>   
>> +	/* Re-use temporally (till 1st modification) the memory */
> 
> So this means that the runtime (modifiable) table
> (pd->runtime_table>state) is mapped to the default (basic) table
> (pd->default_table->state) until the first call to
> em_dev_update_perf_domain() (here mentioned as the 1st modification)?

correct

> 
> IMHO, not easy to understand since neither the cover letter, nor
> documentation patch 15/17 describes this in a consistent story.

I'll add that to the patch header and also to the documentation patch
which is later in the series.