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Date: Wed, 17 Aug 2022 15:21:09 +0100 From: Ionela Voinescu <ionela.voinescu@....com> To: Pierre Gondois <pierre.gondois@....com> Cc: linux-kernel@...r.kernel.org, Lukasz.Luba@....com, Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, linux-doc@...r.kernel.org Subject: Re: [PATCH] sched/topology: Remove EM_MAX_COMPLEXITY limit Hi Pierre, On Friday 12 Aug 2022 at 12:16:19 (+0200), Pierre Gondois wrote: > From: Pierre Gondois <Pierre.Gondois@....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.
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