linux-kernel - Re: [PATCH v2 09/17] PM: EM: Add RCU mechanism which safely cleans the old data

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Message-ID: <2cee7958-8bbc-a7b4-14c6-05ca9770171d@arm.com>
Date:   Mon, 3 Jul 2023 16:49:10 +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 09/17] PM: EM: Add RCU mechanism which safely cleans
 the old data



On 5/30/23 11:02, Dietmar Eggemann wrote:
> On 12/05/2023 11:57, Lukasz Luba wrote:
>> The EM is going to support runtime modifications of the power data.
>> In order to achieve that prepare the internal mechanism. This patch
>> introduces RCU safe mechanism to clean up the old allocated EM data.
> 
> s/In order to achieve that prepare the internal mechanism. This patch
> introduces/Introduce ... much shorter, same information.

OK

> 
> [...]
> 
>> +static void em_perf_runtime_table_set(struct device *dev,
>> +				      struct em_perf_table *runtime_table)
>> +{
>> +	struct em_perf_domain *pd = dev->em_pd;
>> +	struct em_perf_table *tmp;
>> +
>> +	tmp = pd->runtime_table;
>> +
>> +	rcu_assign_pointer(pd->runtime_table, runtime_table);
>> +
>> +	em_cpufreq_update_efficiencies(dev, runtime_table->state);
>> +
>> +	if (trace_em_perf_state_enabled()) {
>> +		unsigned long freq, power, cost, flags;
>> +		int i;
>> +
>> +		for (i = 0; i < pd->nr_perf_states; i++) {
>> +			freq = runtime_table->state[i].frequency;
>> +			power = runtime_table->state[i].power;
>> +			cost = runtime_table->state[i].cost;
>> +			flags = runtime_table->state[i].flags;
>> +
>> +			trace_em_perf_state(dev_name(dev), pd->nr_perf_states,
>> +					    i, freq, power, cost, flags);
>> +		}
>> +	}
>> +
>> +	/*
>> +	 * Check if the 'state' array is not actually the one from setup.
>> +	 * If it is then don't free it.
>> +	 */
>> +	if (tmp->state == pd->table)
> 
> It's unfortunate that you have the refactoring in 13/17 which would lead to:
> 
>   if (pd->runtime_table>state == pd->default_table->state)
>           ^^^^^^^^^^^^^              ^^^^^^^^^^^^^
> 
> so people would immediately grasp one of the core concepts of the
> design: non-modifiable default_table and modifiable runtime_table.
> 
> I still belief that it would be the better idea to do the refactoring first.
> 
> [...]

That make sense. Let me try that approach as see how the patch set would
look like.