Messages in this thread | | | Date | Tue, 4 May 2021 10:27:15 +0100 | From | Vincent Donnefort <> | Subject | Re: [PATCH v2 2/2] sched/fair: Fix negative energy delta in find_energy_efficient_cpu() |
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On Thu, Apr 29, 2021 at 11:19:48AM +0100, Pierre.Gondois@arm.com wrote: > From: Pierre Gondois <Pierre.Gondois@arm.com> > > find_energy_efficient_cpu() (feec()) searches the best energy CPU > to place a task on. To do so, compute_energy() estimates the energy > impact of placing the task on a CPU, based on CPU and task utilization > signals. > > Utilization signals can be concurrently updated while evaluating a > performance domain (pd). In some cases, this leads to having a > 'negative delta', i.e. placing the task in the pd is seen as an > energy gain. Thus, any further energy comparison is biased. > > In case of a 'negative delta', return prev_cpu since: > 1. a 'negative delta' happens in less than 0.5% of feec() calls, > on a Juno with 6 CPUs (4 little, 2 big) > 2. it is unlikely to have two consecutive 'negative delta' for > a task, so if the first call fails, feec() will correctly > place the task in the next feec() call > 3. EAS current behavior tends to select prev_cpu if the task > doesn't raise the OPP of its current pd. prev_cpu is EAS's > generic decision > 4. prev_cpu should be preferred to returning an error code. > In the latter case, select_idle_sibling() would do the placement, > selecting a big (and not energy efficient) CPU. As 3., the task > would potentially reside on the big CPU for a long time > > Reported-by: Xuewen Yan <xuewen.yan@unisoc.com> > Suggested-by: Xuewen Yan <xuewen.yan@unisoc.com> > Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com> > ---
I've been testing this patch on the Google's Pixel4, with a modified kernel that we are using to evalute mailine performance and energy consumption for a "real-life" mobile usage.
As always, I ran the Work2.0 workload from PCMark on Android. With that setup I haven't observed any statistically significant performance change neither any CPU Idle residency modification. Nevertheless, this code protected against ~600 bad computations (and by extent bad placements) during a single PCMark iteration and by looking at the traces, this is saving from spurious wake-ups that would otherwise happen on the biggest CPUs of the system.
+ Reviewed-by: Vincent Donnefort <vincent.donnefort@arm.com>
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