Re: [PATCH v6 11/23] PM: EM: Add API for updating the runtime modifiable EM

[Rafael J. Wysocki]

I don't really like using the API TLA in patch subjects, because it
does not really say much.  IMO a subject like this would be better:

"PM: EM: Introduce em_dev_update_perf_domain() for EM updates"

On Thu, Jan 4, 2024 at 6:15 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
>
> Add API function em_dev_update_perf_domain() which allows to safely
> change the EM.

"... which allows the EM to be changed safely."

New paragraph:

> The concurrent modifiers are protected by the mutex
> to serialize them. Removal of the old memory is asynchronous and
> handled by the RCU mechanisms.

"Concurrent updaters are serialized with a mutex and the removal of
memory that will not be used any more is carried out with the help of
RCU."

>
> Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> ---
>  include/linux/energy_model.h |  8 +++++++
>  kernel/power/energy_model.c  | 41 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
>
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index 753d70d0ce7e..f33257ed83fd 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -183,6 +183,8 @@ struct em_data_callback {
>
>  struct em_perf_domain *em_cpu_get(int cpu);
>  struct em_perf_domain *em_pd_get(struct device *dev);
> +int em_dev_update_perf_domain(struct device *dev,
> +                             struct em_perf_table __rcu *new_table);
>  int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
>                                 struct em_data_callback *cb, cpumask_t *span,
>                                 bool microwatts);
> @@ -376,6 +378,12 @@ struct em_perf_table __rcu *em_allocate_table(struct em_perf_domain *pd)
>         return NULL;
>  }
>  static inline void em_free_table(struct em_perf_table __rcu *table) {}
> +static inline
> +int em_dev_update_perf_domain(struct device *dev,
> +                             struct em_perf_table __rcu *new_table)
> +{
> +       return -EINVAL;
> +}
>  #endif
>
>  #endif
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index bbc406db0be1..496dc00835c6 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -220,6 +220,47 @@ static int em_allocate_perf_table(struct em_perf_domain *pd,
>         return 0;
>  }
>
> +/**
> + * em_dev_update_perf_domain() - Update runtime EM table for a device
> + * @dev                : Device for which the EM is to be updated
> + * @table      : The new EM table that is going to be used from now

This is called "new_table" below.

> + *
> + * Update EM runtime modifiable table for the @dev using the provided @table.
> + *
> + * This function uses mutex to serialize writers, so it must not be called

"uses a mutex"

> + * from non-sleeping context.

"a non-sleeping context".

> + *
> + * Return 0 on success or a proper error in case of failure.

It is not clear what "a proper error" means.  It would be better to
simply say "or an error code on failure" IMO.

> + */
> +int em_dev_update_perf_domain(struct device *dev,
> +                             struct em_perf_table __rcu *new_table)
> +{
> +       struct em_perf_table __rcu *old_table;
> +       struct em_perf_domain *pd;
> +
> +       /* Serialize update/unregister or concurrent updates */
> +       mutex_lock(&em_pd_mutex);
> +
> +       if (!dev || !dev->em_pd) {

dev need not be checked under the lock.

> +               mutex_unlock(&em_pd_mutex);
> +               return -EINVAL;
> +       }
> +       pd = dev->em_pd;
> +
> +       em_table_inc(new_table);
> +
> +       old_table = pd->em_table;
> +       rcu_assign_pointer(pd->em_table, new_table);
> +
> +       em_cpufreq_update_efficiencies(dev, new_table->state);
> +
> +       em_table_dec(old_table);
> +
> +       mutex_unlock(&em_pd_mutex);
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(em_dev_update_perf_domain);
> +
>  static int em_create_runtime_table(struct em_perf_domain *pd)
>  {
>         struct em_perf_table __rcu *table;
> --