Re: [PATCH v4 11/18] PM: EM: Add runtime update interface to modify EM power

[Rafael J. Wysocki]

On Mon, Sep 25, 2023 at 10:11 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:

First off, I would merge this with the previous patch, as the changes
would be much clearer then IMO.

> Add an interface which allows to modify EM power data at runtime.
> The new power information is populated by the provided callback, which
> is called for each performance state.

But it all starts with copying the frequencies from the default table.

> The CPU frequencies' efficiency is
> re-calculated since that might be affected as well. The old EM memory
> is going to be freed later using RCU mechanism.

Not all of it, but the old runtime table that is not going to be used any more.

> Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> ---
>  include/linux/energy_model.h |   8 +++
>  kernel/power/energy_model.c  | 111 +++++++++++++++++++++++++++++++++++
>  2 files changed, 119 insertions(+)
>
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index 740e7c25cfff..8f055ab356ed 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -201,6 +201,8 @@ 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_data_callback *cb,
> +                             void *priv);
>  int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
>                                 struct em_data_callback *cb, cpumask_t *span,
>                                 bool microwatts);
> @@ -384,6 +386,12 @@ static inline int em_pd_nr_perf_states(struct em_perf_domain *pd)
>  {
>         return 0;
>  }
> +static inline
> +int em_dev_update_perf_domain(struct device *dev, struct em_data_callback *cb,
> +                             void *priv)
> +{
> +       return -EINVAL;
> +}
>  #endif
>
>  #endif
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index 2345837bfd2c..78e1495dc87e 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -172,6 +172,101 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>         return 0;
>  }
>
> +/**
> + * em_dev_update_perf_domain() - Update runtime EM table for a device
> + * @dev                : Device for which the EM is to be updated
> + * @cb         : Callback function providing the power data for the EM
> + * @priv       : Pointer to private data useful for passing context
> + *             which might be required while calling @cb

It is still unclear to me who is going to use this priv pointer and how.

> + *
> + * Update EM runtime modifiable table for a @dev using the callback
> + * defined in @cb. The EM new power values are then used for calculating
> + * the em_perf_state::cost for associated performance state.

It actually allocates a new runtime table and populates it from
scratch, using the frequencies from the default table and the
callback.

> + *
> + * This function uses mutex to serialize writers, so it must not be called

"a mutex"

> + * from non-sleeping context.
> + *
> + * Return 0 on success or a proper error in case of failure.
> + */
> +int em_dev_update_perf_domain(struct device *dev, struct em_data_callback *cb,
> +                             void *priv)
> +{
> +       struct em_perf_table *runtime_table;
> +       unsigned long power, freq;
> +       struct em_perf_domain *pd;
> +       int ret, i;
> +
> +       if (!cb || !cb->update_power)
> +               return -EINVAL;
> +
> +       /*
> +        * The lock serializes update and unregister code paths. When the
> +        * EM has been unregistered in the meantime, we should capture that
> +        * when entering this critical section. It also makes sure that
> +        * two concurrent updates will be serialized.
> +        */
> +       mutex_lock(&em_pd_mutex);
> +
> +       if (!dev || !dev->em_pd) {

Checking dev against NULL under the mutex is pointless (either it is
NULL or it isn't, so check it earlier).

> +               ret = -EINVAL;
> +               goto unlock_em;
> +       }
> +
> +       pd = dev->em_pd;

And I would check pd against NULL here.

> +
> +       runtime_table = kzalloc(sizeof(*runtime_table), GFP_KERNEL);
> +       if (!runtime_table) {
> +               ret = -ENOMEM;
> +               goto unlock_em;
> +       }
> +
> +       runtime_table->state = kcalloc(pd->nr_perf_states,
> +                                      sizeof(struct em_perf_state),
> +                                      GFP_KERNEL);
> +       if (!runtime_table->state) {
> +               ret = -ENOMEM;
> +               goto free_runtime_table;
> +       }

The above allocations can be merged into one and allocating memory
under the mutex is questionable.

> +
> +       /* Populate runtime table with updated values using driver callback */
> +       for (i = 0; i < pd->nr_perf_states; i++) {
> +               freq = pd->default_table->state[i].frequency;
> +               runtime_table->state[i].frequency = freq;
> +
> +               /*
> +                * Call driver callback to get a new power value for
> +                * a given frequency.
> +                */
> +               ret = cb->update_power(dev, freq, &power, priv);
> +               if (ret) {
> +                       dev_dbg(dev, "EM: runtime update error: %d\n", ret);
> +                       goto free_runtime_state_table;
> +               }
> +
> +               runtime_table->state[i].power = power;
> +       }
> +
> +       ret = em_compute_costs(dev, runtime_table->state, cb,
> +                              pd->nr_perf_states, pd->flags);
> +       if (ret)
> +               goto free_runtime_state_table;
> +
> +       em_perf_runtime_table_set(dev, runtime_table);
> +
> +       mutex_unlock(&em_pd_mutex);
> +       return 0;
> +
> +free_runtime_state_table:
> +       kfree(runtime_table->state);
> +free_runtime_table:
> +       kfree(runtime_table);
> +unlock_em:
> +       mutex_unlock(&em_pd_mutex);
> +
> +       return -EINVAL;
> +}
> +EXPORT_SYMBOL_GPL(em_dev_update_perf_domain);
> +
>  static int em_create_perf_table(struct device *dev, struct em_perf_domain *pd,
>                                 int nr_states, struct em_data_callback *cb,
>                                 unsigned long flags)
> @@ -494,6 +589,8 @@ void em_dev_unregister_perf_domain(struct device *dev)
>          * The mutex separates all register/unregister requests and protects
>          * from potential clean-up/setup issues in the debugfs directories.
>          * The debugfs directory name is the same as device's name.
> +        * The lock also protects the updater of the runtime modifiable
> +        * EM and this remover.
>          */
>         mutex_lock(&em_pd_mutex);
>
> @@ -501,9 +598,23 @@ void em_dev_unregister_perf_domain(struct device *dev)
>
>         runtime_table = pd->runtime_table;
>
> +       /*
> +        * Safely destroy runtime modifiable EM. By using the call
> +        * synchronize_rcu() we make sure we don't progress till last user
> +        * finished the RCU section and our update got applied.
> +        */
>         rcu_assign_pointer(pd->runtime_table, NULL);
>         synchronize_rcu();
>
> +       /*
> +        * After the sync no updates will be in-flight, so free the
> +        * memory allocated for runtime table (if there was such).
> +        */
> +       if (runtime_table != pd->default_table) {
> +               kfree(runtime_table->state);
> +               kfree(runtime_table);
> +       }

Can't this race with the RCU callback freeing the runtime table?

> +
>         kfree(pd->default_table->state);
>         kfree(pd->default_table);
>         kfree(dev->em_pd);
> --