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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 F3E9DC34026 for ; Tue, 18 Feb 2020 09:54:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D01D320659 for ; Tue, 18 Feb 2020 09:54:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726475AbgBRJyQ (ORCPT ); Tue, 18 Feb 2020 04:54:16 -0500 Received: from foss.arm.com ([217.140.110.172]:48864 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726323AbgBRJyQ (ORCPT ); Tue, 18 Feb 2020 04:54:16 -0500 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 49AB91FB; Tue, 18 Feb 2020 01:54:15 -0800 (PST) Received: from [192.168.0.7] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C04E63F6CF; Tue, 18 Feb 2020 01:54:13 -0800 (PST) Subject: Re: [PATCH 1/3] sched/rt: cpupri_find: implement fallback mechanism for !fit case To: Qais Yousef Cc: Ingo Molnar , Peter Zijlstra , Steven Rostedt , Pavan Kondeti , Juri Lelli , Vincent Guittot , Ben Segall , Mel Gorman , linux-kernel@vger.kernel.org References: <20200214163949.27850-1-qais.yousef@arm.com> <20200214163949.27850-2-qais.yousef@arm.com> <20200217234549.rpv3ns7bd7l6twqu@e107158-lin> From: Dietmar Eggemann Message-ID: <50eee4ae-a733-d8e4-9f57-ab05678545fc@arm.com> Date: Tue, 18 Feb 2020 10:53:55 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200217234549.rpv3ns7bd7l6twqu@e107158-lin> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/02/2020 00:45, Qais Yousef wrote: > On 02/17/20 20:09, Dietmar Eggemann wrote: >> On 14/02/2020 17:39, Qais Yousef wrote: >> >> [...] >> >>> /** >>> * cpupri_find - find the best (lowest-pri) CPU in the system >>> * @cp: The cpupri context >>> @@ -62,80 +115,72 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p, >>> struct cpumask *lowest_mask, >>> bool (*fitness_fn)(struct task_struct *p, int cpu)) >>> { >>> - int idx = 0; >>> int task_pri = convert_prio(p->prio); >>> + int best_unfit_idx = -1; >>> + int idx = 0, cpu; >>> >>> BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES); >>> >>> for (idx = 0; idx < task_pri; idx++) { >>> - struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; >>> - int skip = 0; >>> >>> - if (!atomic_read(&(vec)->count)) >>> - skip = 1; >>> - /* >>> - * When looking at the vector, we need to read the counter, >>> - * do a memory barrier, then read the mask. >>> - * >>> - * Note: This is still all racey, but we can deal with it. >>> - * Ideally, we only want to look at masks that are set. >>> - * >>> - * If a mask is not set, then the only thing wrong is that we >>> - * did a little more work than necessary. >>> - * >>> - * If we read a zero count but the mask is set, because of the >>> - * memory barriers, that can only happen when the highest prio >>> - * task for a run queue has left the run queue, in which case, >>> - * it will be followed by a pull. If the task we are processing >>> - * fails to find a proper place to go, that pull request will >>> - * pull this task if the run queue is running at a lower >>> - * priority. >>> - */ >>> - smp_rmb(); >>> - >>> - /* Need to do the rmb for every iteration */ >>> - if (skip) >>> - continue; >>> - >>> - if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids) >>> + if (!__cpupri_find(cp, p, lowest_mask, idx)) >>> continue; >>> >>> - if (lowest_mask) { >>> - int cpu; >> >> Shouldn't we add an extra condition here? >> >> + if (!static_branch_unlikely(&sched_asym_cpucapacity)) >> + return 1; >> + >> >> Otherwise non-heterogeneous systems have to got through this >> for_each_cpu(cpu, lowest_mask) further below for no good reason. > > Hmm below is the best solution I can think of at the moment. Works for you? > > It's independent of what this patch tries to fix, so I'll add as a separate > patch to the series in the next update. OK. Since we can't set it as early as init_sched_rt_class() root@juno:~# dmesg | grep "\*\*\*" [ 0.501697] *** set sched_asym_cpucapacity <-- CPU cap asym by uArch [ 0.505847] *** init_sched_rt_class() [ 1.796706] *** set sched_asym_cpucapacity <-- CPUfreq kicked in we probably have to do it either by bailing out of cpupri_find() early with this extra condition (above) or by initializing the func pointer dynamically (your example). [...] > @@ -1708,6 +1710,7 @@ static int find_lowest_rq(struct task_struct *task) > struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask); > int this_cpu = smp_processor_id(); > int cpu = task_cpu(task); > + fitness_fn_t fitness_fn; > > /* Make sure the mask is initialized first */ > if (unlikely(!lowest_mask)) > @@ -1716,8 +1719,17 @@ static int find_lowest_rq(struct task_struct *task) > if (task->nr_cpus_allowed == 1) > return -1; /* No other targets possible */ > > + /* > + * Help cpupri_find avoid the cost of looking for a fitting CPU when > + * not really needed. > + */ In case the commend is really needed, for me it would work better logically inverse. [...]