Re: [RFC][PATCH v0.1 3/6] PM: EM: Add special case to em_dev_register_perf_domain()

[Hongyan Xia <hongyan.xia2@arm.com>]

On 08/11/2024 16:38, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> Allow em_dev_register_perf_domain() to register a cost-only stub
> perf domain with one-element states table if the .active_power()
> callback is not provided.
> 
> Subsequently, this will be used by the intel_pstate driver to register
> stub perf domains for CPUs on hybrid systems.
> 
> No intentional functional impact.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>   kernel/power/energy_model.c |   26 +++++++++++++++++++++++---
>   1 file changed, 23 insertions(+), 3 deletions(-)
> 
> Index: linux-pm/kernel/power/energy_model.c
> ===================================================================
> --- linux-pm.orig/kernel/power/energy_model.c
> +++ linux-pm/kernel/power/energy_model.c
> @@ -426,9 +426,11 @@ static int em_create_pd(struct device *d
>   	if (!em_table)
>   		goto free_pd;
>   
> -	ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
> -	if (ret)
> -		goto free_pd_table;
> +	if (cb->active_power) {
> +		ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
> +		if (ret)
> +			goto free_pd_table;
> +	}
>   
>   	ret = em_compute_costs(dev, em_table->state, cb, nr_states, flags);
>   	if (ret)
> @@ -561,11 +563,20 @@ int em_dev_register_perf_domain(struct d
>   {
>   	unsigned long cap, prev_cap = 0;
>   	unsigned long flags = 0;
> +	bool stub_pd = false;
>   	int cpu, ret;
>   
>   	if (!dev || !nr_states || !cb)
>   		return -EINVAL;
>   
> +	if (!cb->active_power) {
> +		if (!cb->get_cost || nr_states > 1 || microwatts)
> +			return -EINVAL;
> +
> +		/* Special case: a stub perf domain. */
> +		stub_pd = true;
> +	}
> +

I wonder if the only purpose of stub_pd is to just skip the capacity 
check below, which doesn't look very nice.

I may be echoing Dietmar's comments here. What's the problem of just 
having 3 domains?

Or, could you just specify the same capacities so that the same-capacity 
check won't fail, but just to use hardware load or CPU pressure to model 
the slight difference in real capacities? This way you'd re-use a lot of 
existing infrastructure.

>   	/*
>   	 * Use a mutex to serialize the registration of performance domains and
>   	 * let the driver-defined callback functions sleep.
> @@ -590,6 +601,15 @@ int em_dev_register_perf_domain(struct d
>   				ret = -EEXIST;
>   				goto unlock;
>   			}
> +
> +			/*
> +			 * The capacity need not be the same for all CPUs in a
> +			 * stub perf domain, so long as the average cost of
> +			 * running on each of them is approximately the same.
> +			 */
> +			if (stub_pd)
> +				continue;
> +
>   			/*
>   			 * All CPUs of a domain must have the same
>   			 * micro-architecture since they all share the same
> 
> 
>