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

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Message-ID: <7469ce56-b46e-54c2-faa6-5b1fddbe86ce@arm.com>
Date:   Mon, 21 Aug 2023 16:30:19 +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,
        mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rafael@...nel.org
Subject: Re: [PATCH v3 09/12] PM: EM: Add RCU mechanism which safely cleans
 the old data



On 8/16/23 14:06, Dietmar Eggemann wrote:
> 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?

You have convinced me. I'll change that code.

> 
> -->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) {
> 
> 
> 

I'll have to modify the unregister function to be aligned with this
approach as well.