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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3EC4EC433EF for ; Tue, 12 Jul 2022 10:53:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232537AbiGLKxP (ORCPT ); Tue, 12 Jul 2022 06:53:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38402 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232503AbiGLKxC (ORCPT ); Tue, 12 Jul 2022 06:53:02 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id BAB0AAEF43 for ; Tue, 12 Jul 2022 03:52:53 -0700 (PDT) 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 E57FB1515; Tue, 12 Jul 2022 03:52:53 -0700 (PDT) Received: from wubuntu (unknown [10.57.86.197]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id ACF8C3F792; Tue, 12 Jul 2022 03:52:51 -0700 (PDT) Date: Tue, 12 Jul 2022 11:52:50 +0100 From: Qais Yousef To: Vincent Guittot Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, david.chen@nutanix.com, zhangqiao22@huawei.com Subject: Re: [PATCH v2] sched/fair: fix case with reduced capacity CPU Message-ID: <20220712105250.axwscy5vna5mkppa@wubuntu> References: <20220708154401.21411-1-vincent.guittot@linaro.org> <20220711160304.njkd3ml7nqpokiim@wubuntu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/11/22 18:42, Vincent Guittot wrote: > On Mon, 11 Jul 2022 at 18:03, Qais Yousef wrote: > > > > Hi Vincent > > > > On 07/08/22 17:44, Vincent Guittot wrote: > > > The capacity of the CPU available for CFS tasks can be reduced because of > > > other activities running on the latter. In such case, it's worth trying to > > > move CFS tasks on a CPU with more available capacity. > > > > > > The rework of the load balance has filtered the case when the CPU is > > > classified to be fully busy but its capacity is reduced. > > > > > > Check if CPU's capacity is reduced while gathering load balance statistic > > > and classify it group_misfit_task instead of group_fully_busy so we can > > > try to move the load on another CPU. > > > > > > Reported-by: David Chen > > > Reported-by: Zhang Qiao > > > Signed-off-by: Vincent Guittot > > > Tested-by: David Chen > > > Tested-by: Zhang Qiao > > > --- > > > > [...] > > > > > @@ -8820,8 +8833,9 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > > > > > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > > > struct rq *rq = cpu_rq(i); > > > + unsigned long load = cpu_load(rq); > > > > > > - sgs->group_load += cpu_load(rq); > > > + sgs->group_load += load; > > > sgs->group_util += cpu_util_cfs(i); > > > sgs->group_runnable += cpu_runnable(rq); > > > sgs->sum_h_nr_running += rq->cfs.h_nr_running; > > > @@ -8851,11 +8865,17 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > > if (local_group) > > > continue; > > > > > > - /* Check for a misfit task on the cpu */ > > > - if (env->sd->flags & SD_ASYM_CPUCAPACITY && > > > - sgs->group_misfit_task_load < rq->misfit_task_load) { > > > - sgs->group_misfit_task_load = rq->misfit_task_load; > > > - *sg_status |= SG_OVERLOAD; > > > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > > > + /* Check for a misfit task on the cpu */ > > > + if (sgs->group_misfit_task_load < rq->misfit_task_load) { > > > + sgs->group_misfit_task_load = rq->misfit_task_load; > > > + *sg_status |= SG_OVERLOAD; > > > + } > > > + } else if ((env->idle != CPU_NOT_IDLE) && > > > + sched_reduced_capacity(rq, env->sd)) { > > > + /* Check for a task running on a CPU with reduced capacity */ > > > + if (sgs->group_misfit_task_load < load) > > > + sgs->group_misfit_task_load = load; > > > } > > > } > > > > Small questions mostly for my education purposes. > > > > The new condition only applies for SMP systems. The reason asym systems don't > > care is because misfit check already considers capacity pressure when checking > > that the task fits_capacity()? > > Yes > > > > > It **seems** to me that the migration margin in fits_capacity() acts like the > > sd->imbalance_pct when check_cpu_capacity() is called by > > sched_reduced_capacity(), did I get it right? > > Yes > > > > > If I got it right, if the migration margin ever tweaked, could we potentially > > start seeing this kind of reported issue on asym systems then? I guess not. It > > just seems to me for asym systems tweaking the migration margin is similar to > > tweaking imbalance_pct for smp ones. But the subtlety is greater as > > imbalance_pct is still used in asym systems. > > You should not because the task will end up being misfit whatever the > margin. The only change would be how fast you will detect and migrate Yes. It's just this control/knob of how fast is different for HMP vs SMP. check_misfit_status() does rely on rq->misfit_task_load AND check_cpu_capacity(). So for that case the 2 knobs will impact how fast this check will converge. I want to say we should document this with a comment, but tbh I have no clue how this can be explained clearly without being way too verbose. All looks good to me anyway. Thanks for clarifying. Cheers -- Qais Yousef