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

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Message-ID: <c0a23406-662d-cb85-e68c-5775de6b9440@arm.com>
Date:   Tue, 30 May 2023 12:02:11 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rafael@...nel.org
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
Subject: Re: [PATCH v2 09/17] PM: EM: Add RCU mechanism which safely cleans
 the old data

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.

[...]

> +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.

[...]