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Date: Tue, 8 Mar 2022 18:19:50 +0000 From: Vincent Donnefort <vincent.donnefort@....com> To: peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org Cc: linux-kernel@...r.kernel.org, dietmar.eggemann@....com, morten.rasmussen@....com, chris.redpath@....com, qperret@...gle.com, Vincent Donnefort <vincent.donnefort@....com> Subject: [PATCH v3 0/7] feec() energy margin removal find_energy_efficient() (feec()) will migrate a task to save energy only if it saves at least 6% of the total energy consumed by the system. This conservative approach is a problem on a system where a lot of small tasks create a huge load on the overall: very few of them will be allowed to migrate to a smaller CPU, wasting a lot of energy. Instead of trying to determine yet another margin, let's try to remove it. The first elements of this patch-set are various fixes and improvement that stabilizes task_util and ensures energy comparison fairness across all CPUs of the topology. Only once those fixed, we can completely remove the margin and let feec() aggressively place task and save energy. This has been validated by two different ways: First using LISA's eas_behaviour test suite. This is composed of a set of scenario and verify if the task placement is optimum. No failure have been observed and it also improved some tests such as Ramp-Down (as the placement is now more energy oriented) and *ThreeSmall (as no bouncing between clusters happen anymore). * Hikey960: 100% PASSED * DB-845C: 100% PASSED * RB5: 100% PASSED Second, using an Android benchmark: PCMark2 on a Pixel4, with a lot of backports to have a scheduler as close as we can from mainline. +------------+-----------------+-----------------+ | Test | Perf | Energy [1] | +------------+-----------------+-----------------+ | Web2 | -0.3% pval 0.03 | -1.8% pval 0.00 | | Video2 | -0.3% pval 0.13 | -5.6% pval 0.00 | | Photo2 [2] | -3.8% pval 0.00 | -1% pval 0.00 | | Writing2 | 0% pval 0.13 | -1% pval 0.00 | | Data2 | 0% pval 0.8 | -0.43 pval 0.00 | +------------+-----------------+-----------------+ The margin removal let the kernel make the best use of the Energy Model, tasks are more likely to be placed where they fit and this saves a substantial amount of energy, while having a limited impact on performances. [1] This is an energy estimation based on the CPU activity and the Energy Model for this device. "All models are wrong but some are useful"; yes, this is an imperfect estimation that doesn't take into account some idle states and shared power rails. Nonetheless this is based on the information the kernel has during runtime and it proves the scheduler can take better decisions based solely on those data. [2] This is the only performance impact observed. The debugging of this test showed no issue with task placement. The better score was solely due to some critical threads held on better performing CPUs. If a thread needs a higher capacity CPU, the placement must result from a user input (with e.g. uclamp min) instead of being artificially held on less efficient CPUs by feec(). Notice also, the experiment didn't use the Android only latency_sensitive feature which would hide this problem on a real-life device. v2 -> v3: - feec(): introduce energy_env struct (Dietmar) - PELT migration decay: Only apply when src CPU is idle (Vincent G.) - PELT migration decay: Do not apply when cfs_rq is throttled - PELT migration decay: Snapshot the lag at cfs_rq's level v1 -> v2: - Fix PELT migration last_update_time (previously root cfs_rq's). - Add Dietmar's patches to refactor feec()'s CPU loop. - feec(): renaming busy time functions get_{pd,tsk}_busy_time() - feec(): pd_cap computation in the first for_each_cpu loop. - feec(): create get_pd_max_util() function (previously within compute_energy()) - feec(): rename base_energy_pd to base_energy. Dietmar Eggemann (3): sched, drivers: Remove max param from effective_cpu_util()/sched_cpu_util() sched/fair: Rename select_idle_mask to select_rq_mask sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu() Vincent Donnefort (4): sched/fair: Provide u64 read for 32-bits arch helper sched/fair: Decay task PELT values during migration sched/fair: Remove task_util from effective utilization in feec() sched/fair: Remove the energy margin in feec() drivers/powercap/dtpm_cpu.c | 33 +-- drivers/thermal/cpufreq_cooling.c | 6 +- include/linux/sched.h | 2 +- kernel/sched/core.c | 15 +- kernel/sched/cpufreq_schedutil.c | 5 +- kernel/sched/fair.c | 376 ++++++++++++++++++------------ kernel/sched/sched.h | 49 +++- 7 files changed, 295 insertions(+), 191 deletions(-) -- 2.25.1
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