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=-16.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,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 046DDC433B4 for ; Thu, 29 Apr 2021 10:20:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BFB3461456 for ; Thu, 29 Apr 2021 10:20:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240419AbhD2KVW (ORCPT ); Thu, 29 Apr 2021 06:21:22 -0400 Received: from foss.arm.com ([217.140.110.172]:46170 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240322AbhD2KU4 (ORCPT ); Thu, 29 Apr 2021 06:20:56 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CDB1E106F; Thu, 29 Apr 2021 03:20:09 -0700 (PDT) Received: from e120189.arm.com (unknown [10.57.56.207]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 9DAC63F70D; Thu, 29 Apr 2021 03:20:06 -0700 (PDT) From: Pierre.Gondois@arm.com To: linux-kernel@vger.kernel.org, xuewen.yan@unisoc.com, qperret@qperret.net, dietmar.eggemann@arm.com Cc: Lukasz.Luba@arm.com, Vincent.Donnefort@arm.com, Pierre Gondois , mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com Subject: [PATCH v2 2/2] sched/fair: Fix negative energy delta in find_energy_efficient_cpu() Date: Thu, 29 Apr 2021 11:19:48 +0100 Message-Id: <20210429101948.31224-3-Pierre.Gondois@arm.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20210429101948.31224-1-Pierre.Gondois@arm.com> References: <20210429101948.31224-1-Pierre.Gondois@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Pierre Gondois 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 Suggested-by: Xuewen Yan Signed-off-by: Pierre Gondois --- kernel/sched/fair.c | 28 ++++++++++++++++------------ 1 file changed, 16 insertions(+), 12 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c5351366fb93..935f1ea267a9 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6594,15 +6594,15 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) { unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX; struct root_domain *rd = cpu_rq(smp_processor_id())->rd; + int cpu, best_energy_cpu = prev_cpu, target = -1; unsigned long cpu_cap, util, base_energy = 0; - int cpu, best_energy_cpu = prev_cpu; struct sched_domain *sd; struct perf_domain *pd; rcu_read_lock(); pd = rcu_dereference(rd->pd); if (!pd || READ_ONCE(rd->overutilized)) - goto fail; + goto unlock; /* * Energy-aware wake-up happens on the lowest sched_domain starting @@ -6612,7 +6612,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd))) sd = sd->parent; if (!sd) - goto fail; + goto unlock; + + target = prev_cpu; sync_entity_load_avg(&p->se); if (!task_util_est(p)) @@ -6666,6 +6668,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) if (compute_prev_delta) { prev_delta = compute_energy(p, prev_cpu, pd); + if (prev_delta < base_energy_pd) + goto unlock; prev_delta -= base_energy_pd; best_delta = min(best_delta, prev_delta); } @@ -6673,6 +6677,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) /* Evaluate the energy impact of using this CPU. */ if (max_spare_cap_cpu >= 0) { cur_delta = compute_energy(p, max_spare_cap_cpu, pd); + if (cur_delta < base_energy_pd) + goto unlock; cur_delta -= base_energy_pd; if (cur_delta < best_delta) { best_delta = cur_delta; @@ -6680,25 +6686,23 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) } } } -unlock: + rcu_read_unlock(); /* * Pick the best CPU if prev_cpu cannot be used, or if it saves at * least 6% of the energy used by prev_cpu. */ - if (prev_delta == ULONG_MAX) - return best_energy_cpu; - - if ((prev_delta - best_delta) > ((prev_delta + base_energy) >> 4)) - return best_energy_cpu; + if ((prev_delta == ULONG_MAX) || + (prev_delta - best_delta) > ((prev_delta + base_energy) >> 4)) + target = best_energy_cpu; - return prev_cpu; + return target; -fail: +unlock: rcu_read_unlock(); - return -1; + return target; } /* -- 2.17.1