On Thursday 25 Feb 2021 at 08:36:11 (+0000), [email protected] wrote: > From: Vincent Donnefort <[email protected]> > > find_energy_efficient_cpu() (feec()) computes for each perf_domain (pd) an > energy delta as follows: > > feec(task) > for_each_pd > base_energy = compute_energy(task, -1, pd) > -> for_each_cpu(pd) > -> cpu_util_next(cpu, task, -1) > > energy_delta = compute_energy(task, dst_cpu, pd) > -> for_each_cpu(pd) > -> cpu_util_next(cpu, task, dst_cpu) > energy_delta -= base_energy > > Then it picks the best CPU as being the one that minimizes energy_delta. > > cpu_util_next() estimates the CPU utilization that would happen if the > task was placed on dst_cpu as follows: > > max(cpu_util + task_util, cpu_util_est + _task_util_est) > > The task contribution to the energy delta can then be either: > > (1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0 > and _task_util_est > cpu_util. > (2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est. > > (cpu_util_est doesn't appear here. It is 0 when a CPU is idle and > otherwise must be small enough so that feec() takes the CPU as a > potential target for the task placement) > > This is problematic for feec(), as cpu_util_next() might give an unfair > advantage to a CPU which is mostly busy (2) compared to one which is > mostly idle (1). _task_util_est being always bigger than task_util in > feec() (as the task is waking up), the task contribution to the energy > might look smaller on certain CPUs (2) and this breaks the energy > comparison. > > This issue is, moreover, not sporadic. By starving idle CPUs, it keeps > their cpu_util < _task_util_est (1) while others will maintain cpu_util > > _task_util_est (2). > > Fix this problem by always using max(task_util, _task_util_est) as a task > contribution to the energy (ENERGY_UTIL). The new estimated CPU > utilization for the energy would then be: > > max(cpu_util, cpu_util_est) + max(task_util, _task_util_est) > > compute_energy() still needs to know which OPP would be selected if the > task would be migrated in the perf_domain (FREQUENCY_UTIL). Hence, > cpu_util_next() is still used to estimate the maximum util within the pd. > > Signed-off-by: Vincent Donnefort <[email protected]>
Reviewed-by: Quentin Perret <[email protected]> Thanks, Quentin
