On 28/10/2022 17:30, Pierre Gondois wrote:
> From: Pierre Gondois <Pierre.Gondois@xxxxxxx>
>
> 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@xxxxxxx>
> Reviewed-by: Lukasz Luba <lukasz.luba@xxxxxxx>
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.
[...]
