linux-kernel - Re: [PATCH v3 09/12] PM: EM: Add RCU mechanism which safely cleans the old data

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Date:   Wed, 16 Aug 2023 15:06:30 +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,
        mhiramat@...nel.org
Subject: Re: [PATCH v3 09/12] PM: EM: Add RCU mechanism which safely cleans
 the old data

On 21/07/2023 17:50, Lukasz Luba wrote:
> The EM is going to support runtime modifications of the power data.
> Introduce RCU safe mechanism to clean up the old allocated EM data.
> It also adds a mutex for the EM structure to serialize the modifiers.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  kernel/power/energy_model.c | 42 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index c2f8a0046f8a..4596bfe7398e 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -23,6 +23,9 @@
>   */
>  static DEFINE_MUTEX(em_pd_mutex);
>  
> +static void em_cpufreq_update_efficiencies(struct device *dev,
> +					   struct em_perf_state *table);
> +
>  static bool _is_cpu_device(struct device *dev)
>  {
>  	return (dev->bus == &cpu_subsys);
> @@ -104,6 +107,45 @@ static void em_debug_create_pd(struct device *dev) {}
>  static void em_debug_remove_pd(struct device *dev) {}
>  #endif
>  
> +static void em_destroy_rt_table_rcu(struct rcu_head *rp)
> +{
> +	struct em_perf_table *runtime_table;
> +
> +	runtime_table = container_of(rp, struct em_perf_table, rcu);
> +	kfree(runtime_table->state);
> +	kfree(runtime_table);
> +}
> +
> +static void em_destroy_tmp_setup_rcu(struct rcu_head *rp)
> +{
> +	struct em_perf_table *runtime_table;
> +
> +	runtime_table = container_of(rp, struct em_perf_table, rcu);
> +	kfree(runtime_table);
> +}

Still don't like that we have to have 2 rcu callbacks here. In case we
could assign default_table to runtime_table in em_create_pd() (and not
just default_table->state to runtime_table->state) IMHO we would only
need one rcu callback?

-->8--

-static void em_destroy_tmp_setup_rcu(struct rcu_head *rp)
-{
-       struct em_perf_table *runtime_table;
-
-       runtime_table = container_of(rp, struct em_perf_table, rcu);
-       kfree(runtime_table);
-}
-
 static void em_perf_runtime_table_set(struct device *dev,
                                      struct em_perf_table *runtime_table)
 {
@@ -136,13 +128,8 @@ static void em_perf_runtime_table_set(struct device *dev,
 
        em_cpufreq_update_efficiencies(dev, runtime_table->state);
 
-       /*
-        * Check if the 'state' array is not actually the one from setup.
-        * If it is then don't free it.
-        */
-       if (tmp->state == pd->default_table->state)
-               call_rcu(&tmp->rcu, em_destroy_tmp_setup_rcu);
-       else
+       /* Don't free default table (inital value of runtime table) */
+       if (tmp != pd->default_table)
                call_rcu(&tmp->rcu, em_destroy_rt_table_rcu);
 }
 
@@ -349,7 +336,6 @@ static int em_create_pd(struct device *dev, int nr_states,
                        unsigned long flags)
 {
        struct em_perf_table *default_table;
-       struct em_perf_table *runtime_table;
        struct em_perf_domain *pd;
        struct device *cpu_dev;
        int cpu, ret, num_cpus;
@@ -382,24 +368,15 @@ static int em_create_pd(struct device *dev, int nr_states,
 
        pd->default_table = default_table;
 
-       runtime_table = kzalloc(sizeof(*runtime_table), GFP_KERNEL);
-       if (!runtime_table) {
-               kfree(default_table);
-               kfree(pd);
-               return -ENOMEM;
-       }
-
        ret = em_create_perf_table(dev, pd, nr_states, cb, flags);
        if (ret) {
                kfree(default_table);
-               kfree(runtime_table);
                kfree(pd);
                return ret;
        }
 
-       /* Re-use temporally (till 1st modification) the memory */
-       runtime_table->state = default_table->state;
-       rcu_assign_pointer(pd->runtime_table, runtime_table);
+       /* Initialize runtime table as default table */
+       rcu_assign_pointer(pd->runtime_table, default_table);
 
        if (_is_cpu_device(dev))
                for_each_cpu(cpu, cpus) {