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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 EEC4DC433E0 for ; Wed, 3 Feb 2021 18:44:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 937B864F7E for ; Wed, 3 Feb 2021 18:44:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232863AbhBCSoJ (ORCPT ); Wed, 3 Feb 2021 13:44:09 -0500 Received: from foss.arm.com ([217.140.110.172]:45052 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233035AbhBCSnz (ORCPT ); Wed, 3 Feb 2021 13:43:55 -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 064811396; Wed, 3 Feb 2021 10:43:10 -0800 (PST) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A85613F719; Wed, 3 Feb 2021 10:43:08 -0800 (PST) From: Valentin Schneider To: Qais Yousef Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Morten Rasmussen , Quentin Perret , Pavan Kondeti , Rik van Riel Subject: Re: [PATCH 6/8] sched/fair: Filter out locally-unsolvable misfit imbalances In-Reply-To: <20210203151619.3sa42rqo7eihlfcz@e107158-lin> References: <20210128183141.28097-1-valentin.schneider@arm.com> <20210128183141.28097-7-valentin.schneider@arm.com> <20210203151619.3sa42rqo7eihlfcz@e107158-lin> User-Agent: Notmuch/0.21 (http://notmuchmail.org) Emacs/26.3 (x86_64-pc-linux-gnu) Date: Wed, 03 Feb 2021 18:43:06 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/02/21 15:16, Qais Yousef wrote: > On 01/28/21 18:31, Valentin Schneider wrote: >> Consider the following (hypothetical) asymmetric CPU capacity topology, >> with some amount of capacity pressure (RT | DL | IRQ | thermal): >> >> DIE [ ] >> MC [ ][ ] >> 0 1 2 3 >> >> | CPU | capacity_orig | capacity | >> |-----+---------------+----------| >> | 0 | 870 | 860 | >> | 1 | 870 | 600 | >> | 2 | 1024 | 850 | >> | 3 | 1024 | 860 | >> >> If CPU1 has a misfit task, then CPU0, CPU2 and CPU3 are valid candidates to >> grant the task an uplift in CPU capacity. Consider CPU0 and CPU3 as >> sufficiently busy, i.e. don't have enough spare capacity to accommodate >> CPU1's misfit task. This would then fall on CPU2 to pull the task. > > I think this scenario would be hard in practice, but not impossible. Maybe > gaming could push the system that hard. > Actually I wouldn't be surprised if a moderatly busy Android environment could hit this - slight thermal pressure on the bigs, RT pressure because we know folks love (ab)using RT, a pinch of IRQs in the mix... >> @@ -8450,11 +8457,21 @@ static inline void update_sg_lb_stats(struct lb_env *env, >> continue; >> >> /* Check for a misfit task on the cpu */ >> - if (sd_has_asym_cpucapacity(env->sd) && >> - sgs->group_misfit_task_load < rq->misfit_task_load) { >> - sgs->group_misfit_task_load = rq->misfit_task_load; >> - *sg_status |= SG_OVERLOAD; >> - } >> + if (!sd_has_asym_cpucapacity(env->sd) || >> + !rq->misfit_task_load) >> + continue; >> + >> + *sg_status |= SG_OVERLOAD; >> + sgs->group_has_misfit_task = true; >> + >> + /* >> + * Don't attempt to maximize load for misfit tasks that can't be >> + * granted a CPU capacity uplift. >> + */ >> + if (cpu_capacity_greater(env->dst_cpu, i)) >> + sgs->group_misfit_task_load = max( >> + sgs->group_misfit_task_load, >> + rq->misfit_task_load); > > nit: missing curly braces around the if. > Ack. >> @@ -8504,7 +8521,7 @@ static bool update_sd_pick_busiest(struct lb_env *env, >> /* Don't try to pull misfit tasks we can't help */ >> if (static_branch_unlikely(&sched_asym_cpucapacity) && >> sgs->group_type == group_misfit_task && >> - (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || >> + (!sgs->group_misfit_task_load || >> sds->local_stat.group_type != group_has_spare)) >> return false; >> >> @@ -9464,15 +9481,18 @@ static struct rq *find_busiest_queue(struct lb_env *env, >> case migrate_misfit: >> /* >> * For ASYM_CPUCAPACITY domains with misfit tasks we >> - * simply seek the "biggest" misfit task. >> + * simply seek the "biggest" misfit task we can >> + * accommodate. >> */ >> + if (!cpu_capacity_greater(env->dst_cpu, i)) >> + continue; > > Both this hunk and the one above mean we will end up searching harder to pull > the task into the right cpu taking actual capacity into account. Which is > a good improvement. > Note that those extra checks are to make sure we *don't* downmigrate tasks (as stated somewhere above, this change lets find_busiest_queue() iterate over CPUs bigger than the local CPU's, which wasn't the case before). A "big" CPU will still get the chance to pull a "medium" task, even if a "medium" CPU would have been a "better" choice.