Re: [PATCH v2] sched/topology: Remove EM_MAX_COMPLEXITY limit

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 28/10/2022 17:30, 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]
> 
>   \_ eenv_pd_busy_time(pd) [2]
> 	\_ for_each_cpu(pd)
> 
>   // compute_energy(pd) without the task
>   \_ eenv_pd_max_util(pd, -1) [3.0]
>     \_ for_each_cpu(pd)
>   \_ em_cpu_energy(pd, -1)
>     \_ for_each_ps(pd)
> 
>   // compute_energy(pd) with the task on prev_cpu
>   \_ eenv_pd_max_util(pd, prev_cpu) [3.1]
>     \_ for_each_cpu(pd)
>   \_ em_cpu_energy(pd, prev_cpu)
>     \_ for_each_ps(pd)
> 
>   // compute_energy(pd) with the task on max_spare_cap_cpu
>   \_ eenv_pd_max_util(pd, max_spare_cap_cpu) [3.2]
>     \_ for_each_cpu(pd)
>   \_ em_cpu_energy(pd, max_spare_cap_cpu)
>     \_ for_each_ps(pd)
> 
> [3.1] happens only once since prev_cpu is unique. With the same
> definitions for nr_pd, nr_cpus and nr_ps, the complexity is of:
> nr_pd * (2 * [nr_cpus in pd] + 2 * ([nr_cpus in pd] + [nr_ps in pd]))
> + ([nr_cpus in pd] + [nr_ps in pd])
> 
>  [0]  * (     [1] + [2]      +       [3.0] + [3.2]                  )
> + [3.1]
> 
> = nr_pd * (4 * [nr_cpus in pd] + 2 * [nr_ps in pd])
> + [nr_cpus in prev pd] + nr_ps
> 
> The complexity limit was set to 2048 in:
> commit b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate
> platforms")
> to make "EAS usable up to 16 CPUs with per-CPU DVFS and less than 8
> performance states each". For the same platform, the complexity would
> actually be of:
> 16 * (4 + 2 * 7) + 1 + 7 = 296
> 
> Since the EAS complexity was greatly reduced, bigger platforms can
> handle EAS. For instance, a platform with 112 CPUs with 7 performance
> states each would not reach it:
> 112 * (4 + 2 * 7) + 1 + 7 = 2024
> 
> To reflect this improvement, remove the EAS complexity check.
> 
> Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
> Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

OK, let's remove the specific EAS EM complexity check in this case.

But we should still have some info about the decission that EAS is now
only constraint by EM's own EM_MAX_NUM_CPUS in terms of complexity.

So maybe replace `6.3 - Energy Model complexity` by:

EAS does not impose any complexity limit on numbers of CPUs but relies
on EM's own EM_MAX_NUM_CPUS.

And also mention this fact in the patch-header for future reference
regarding this change.

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