Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy_present static key

[Dietmar Eggemann]

On 08/20/2018 02:44 AM, Quentin Perret wrote:
> In order to ensure a minimal performance impact on non-energy-aware
> systems, introduce a static_key guarding the access to Energy-Aware
> Scheduling (EAS) code.
>
> The static key is set iff all the following conditions are met for at
> least one root domain:
>    1. all online CPUs of the root domain are covered by the Energy
>       Model (EM);
>    2. the complexity of the root domain's EM is low enough to keep
>       scheduling overheads low;
>    3. the root domain has an asymmetric CPU capacity topology (detected
>       by looking for the SD_ASYM_CPUCAPACITY flag in the sched_domain
>       hierarchy).

This is pretty much the list (+ is schedutil running) of conditions to 
set rd->pd != NULL in build_perf_domains().

So when testing 'static_branch_unlikely(&sched_energy_present) && 
rcu_dereference(rd->pd)' don't you test two times the same thing?

Also, if let's say somebody wants to run another EM user (e.g. a thermal 
governor, like IPA) but not EAS on a asymmetric CPU capacity system. 
This can't be achieved with the current static branch approach

So what about using a (disabled by default ?) sched_feature + rd->pd != 
NULL instead?

[...]