From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 44D24C4321D for ; Mon, 20 Aug 2018 09:45:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F0A702098B for ; Mon, 20 Aug 2018 09:45:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F0A702098B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726883AbeHTNAU (ORCPT ); Mon, 20 Aug 2018 09:00:20 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:35126 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726010AbeHTNAU (ORCPT ); Mon, 20 Aug 2018 09:00:20 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AAC1D15B2; Mon, 20 Aug 2018 02:45:25 -0700 (PDT) Received: from queper01-lin.local (queper01-lin.emea.arm.com [10.4.13.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9307D3F2EA; Mon, 20 Aug 2018 02:45:21 -0700 (PDT) From: Quentin Perret To: peterz@infradead.org, rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org Cc: gregkh@linuxfoundation.org, mingo@redhat.com, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, patrick.bellasi@arm.com, valentin.schneider@arm.com, vincent.guittot@linaro.org, thara.gopinath@linaro.org, viresh.kumar@linaro.org, tkjos@google.com, joel@joelfernandes.org, smuckle@google.com, adharmap@codeaurora.org, skannan@codeaurora.org, pkondeti@codeaurora.org, juri.lelli@redhat.com, edubezval@gmail.com, srinivas.pandruvada@linux.intel.com, currojerez@riseup.net, javi.merino@kernel.org, quentin.perret@arm.com Subject: [PATCH v6 12/14] sched/fair: Select an energy-efficient CPU on task wake-up Date: Mon, 20 Aug 2018 10:44:18 +0100 Message-Id: <20180820094420.26590-13-quentin.perret@arm.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20180820094420.26590-1-quentin.perret@arm.com> References: <20180820094420.26590-1-quentin.perret@arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If an Energy Model (EM) is available and if the system isn't overutilized, re-route waking tasks into an energy-aware placement algorithm. The selection of an energy-efficient CPU for a task is achieved by estimating the impact on system-level active energy resulting from the placement of the task on the CPU with the highest spare capacity in each performance domain. This strategy spreads tasks in a performance domain and avoids overly aggressive task packing. The best CPU energy-wise is then selected if it saves a large enough amount of energy with respect to prev_cpu. Although it has already shown significant benefits on some existing targets, this approach cannot scale to platforms with numerous CPUs. This is an attempt to do something useful as writing a fast heuristic that performs reasonably well on a broad spectrum of architectures isn't an easy task. As such, the scope of usability of the energy-aware wake-up path is restricted to systems with the SD_ASYM_CPUCAPACITY flag set, and where the EM isn't too complex. Cc: Ingo Molnar Cc: Peter Zijlstra Signed-off-by: Quentin Perret --- kernel/sched/fair.c | 138 +++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 135 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c4e34368795b..7b311ff0294b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6339,6 +6339,113 @@ static long compute_energy(struct task_struct *p, int dst_cpu, return energy; } +/* + * find_energy_efficient_cpu(): Find most energy-efficient target CPU for the + * waking task. find_energy_efficient_cpu() looks for the CPU with maximum + * spare capacity in each performance domain and uses it as a potential + * candidate to execute the task. Then, it uses the Energy Model to figure + * out which of the CPU candidates is the most energy-efficient. + * + * The rationale for this heuristic is as follows. In a performance domain, + * all the most energy efficient CPU candidates (according to the Energy + * Model) are those for which we'll request a low frequency. When there are + * several CPUs for which the frequency request will be the same, we don't + * have enough data to break the tie between them, because the Energy Model + * only includes active power costs. With this model, if we assume that + * frequency requests follow utilization (e.g. using schedutil), the CPU with + * the maximum spare capacity in a performance domain is guaranteed to be among + * the best candidates of the performance domain. + * + * In practice, it could be preferable from an energy standpoint to pack + * small tasks on a CPU in order to let other CPUs go in deeper idle states, + * but that could also hurt our chances to go cluster idle, and we have no + * ways to tell with the current Energy Model if this is actually a good + * idea or not. So, find_energy_efficient_cpu() basically favors + * cluster-packing, and spreading inside a cluster. That should at least be + * a good thing for latency, and this is consistent with the idea that most + * of the energy savings of EAS come from the asymmetry of the system, and + * not so much from breaking the tie between identical CPUs. That's also the + * reason why EAS is enabled in the topology code only for systems where + * SD_ASYM_CPUCAPACITY is set. + */ +static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu, + struct perf_domain *pd) +{ + unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX; + int cpu, best_energy_cpu = prev_cpu; + struct perf_domain *head = pd; + unsigned long cpu_cap, util; + struct sched_domain *sd; + + sync_entity_load_avg(&p->se); + + if (!task_util_est(p)) + return prev_cpu; + + /* + * Energy-aware wake-up happens on the lowest sched_domain starting + * from sd_asym_cpucapacity spanning over this_cpu and prev_cpu. + */ + sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity)); + while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd))) + sd = sd->parent; + if (!sd) + return prev_cpu; + + while (pd) { + unsigned long cur_energy, spare_cap, max_spare_cap = 0; + int max_spare_cap_cpu = -1; + + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) { + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) + continue; + + /* Skip CPUs that will be overutilized. */ + util = cpu_util_next(cpu, p, cpu); + cpu_cap = capacity_of(cpu); + if (cpu_cap * 1024 < util * capacity_margin) + continue; + + /* Always use prev_cpu as a candidate. */ + if (cpu == prev_cpu) { + prev_energy = compute_energy(p, prev_cpu, head); + if (prev_energy < best_energy) + best_energy = prev_energy; + continue; + } + + /* + * Find the CPU with the maximum spare capacity in + * the performance domain + */ + spare_cap = cpu_cap - util; + if (spare_cap > max_spare_cap) { + max_spare_cap = spare_cap; + max_spare_cap_cpu = cpu; + } + } + + /* Evaluate the energy impact of using this CPU. */ + if (max_spare_cap_cpu >= 0) { + cur_energy = compute_energy(p, max_spare_cap_cpu, head); + if (cur_energy < best_energy) { + best_energy = cur_energy; + best_energy_cpu = max_spare_cap_cpu; + } + } + pd = pd->next; + } + + /* + * Pick the best CPU only if it saves at least 6% of the + * energy used by prev_cpu. + */ + if ((prev_energy - best_energy) > (prev_energy >> 4)) + return best_energy_cpu; + + return prev_cpu; +} + /* * select_task_rq_fair: Select target runqueue for the waking task in domains * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, @@ -6360,13 +6467,37 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f int want_affine = 0; int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); + rcu_read_lock(); if (sd_flag & SD_BALANCE_WAKE) { record_wakee(p); - want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) - && cpumask_test_cpu(cpu, &p->cpus_allowed); + + /* + * Forkees are not accepted in the energy-aware wake-up path + * because they don't have any useful utilization data yet and + * it's not possible to forecast their impact on energy + * consumption. Consequently, they will be placed by + * find_idlest_cpu() on the least loaded CPU, which might turn + * out to be energy-inefficient in some use-cases. The + * alternative would be to bias new tasks towards specific types + * of CPUs first, or to try to infer their util_avg from the + * parent task, but those heuristics could hurt other use-cases + * too. So, until someone finds a better way to solve this, + * let's keep things simple by re-using the existing slow path. + */ + if (static_branch_unlikely(&sched_energy_present)) { + struct root_domain *rd = cpu_rq(cpu)->rd; + struct perf_domain *pd = rcu_dereference(rd->pd); + + if (pd && !READ_ONCE(rd->overutilized)) { + new_cpu = find_energy_efficient_cpu(p, prev_cpu, pd); + goto unlock; + } + } + + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) && + cpumask_test_cpu(cpu, &p->cpus_allowed); } - rcu_read_lock(); for_each_domain(cpu, tmp) { if (!(tmp->flags & SD_LOAD_BALANCE)) break; @@ -6401,6 +6532,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f if (want_affine) current->recent_used_cpu = cpu; } +unlock: rcu_read_unlock(); return new_cpu; -- 2.17.1