Re: [PATCH] sched/topology: Remove EM_MAX_COMPLEXITY limit

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi Pierre,

On Friday 12 Aug 2022 at 12:16:19 (+0200), Pierre Gondois wrote:
> From: Pierre Gondois <Pierre.Gondois@arm.com>
> 
> The Energy Aware Scheduler (EAS) estimates the energy consumption
> of placing a task on different CPUs. The goal is to minimize this
> energy consumption. Estimating the energy of different task placements
> is increasingly complex with the size of the platform. To avoid having
> a slow wake-up path, EAS is only enabled if this complexity is low
> enough.
> 
> The current complexity limit was set in:
> commit b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate
> platforms").
> base on the first implementation of EAS, which was re-computing
> the power of the whole platform for each task placement scenario, cf:
> commit 390031e4c309 ("sched/fair: Introduce an energy estimation helper
> function").
> but the complexity of EAS was reduced in:
> commit eb92692b2544d ("sched/fair: Speed-up energy-aware wake-ups")
> and find_energy_efficient_cpu() (feec) algorithm was updated in:
> commit 3e8c6c9aac42 ("sched/fair: Remove task_util from effective
> utilization in feec()")
> 
> find_energy_efficient_cpu() (feec) is now doing:
> feec()
> \_ for_each_pd(pd) [0]
>   // get max_spare_cap_cpu and compute_prev_delta
>   \_ for_each_cpu(pd) [1]
> 
>   \_ get_pd_busy_time(pd) [2]
>     \_ for_each_cpu(pd)
> 
>   // evaluate pd energy without the task
>   \_ get_pd_max_util(pd, -1) [3.0]
>     \_ for_each_cpu(pd)
>   \_ compute_energy(pd, -1)
>     \_ for_each_ps(pd)
> 
>   // evaluate pd energy with the task on prev_cpu
>   \_ get_pd_max_util(pd, prev_cpu) [3.1]
>     \_ for_each_cpu(pd)
>   \_ compute_energy(pd, prev_cpu)
>     \_ for_each_ps(pd)
> 
>   // evaluate pd energy with the task on max_spare_cap_cpu
>   \_ get_pd_max_util(pd, max_spare_cap_cpu) [3.2]
>     \_ for_each_cpu(pd)
>   \_ compute_energy(pd, max_spare_cap_cpu)
>     \_ for_each_ps(pd)
> 
> [3.1] happens only once since prev_cpu is unique. To have an upper
> bound of the complexity, [3.1] is taken into account for all pds.
> So with the same definitions for nr_pd, nr_cpus and nr_ps,
> the complexity is of:
> nr_pd * (2 * [nr_cpus in pd] + 3 * ([nr_cpus in pd] + [nr_ps in pd]))
>  [0]  * (     [1] + [2]      +       [3.0] + [3.1] + [3.2]          )
> = 5 * nr_cpus + 3 * nr_ps
> 

I just want to draw your attention to [1] and the fact that the
structure of the function changed. Your calculations largely remain the
same - 3 calls to compute_energy() which in turn now calls
eenv_pd_max_util() with operations for each cpu, plus some scattered
calls to eenv_pd_busy_time(), all for each pd.

[1]
https://lore.kernel.org/lkml/20220621090414.433602-7-vdonnefort@google.com/

Thanks,
Ionela.