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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 D16A5C33CB2 for ; Wed, 29 Jan 2020 03:53:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 99B89207FF for ; Wed, 29 Jan 2020 03:53:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="key not found in DNS" (0-bit key) header.d=mg.codeaurora.org header.i=@mg.codeaurora.org header.b="NqBj4uIh" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726756AbgA2DxI (ORCPT ); Tue, 28 Jan 2020 22:53:08 -0500 Received: from mail26.static.mailgun.info ([104.130.122.26]:43655 "EHLO mail26.static.mailgun.info" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726338AbgA2DxI (ORCPT ); Tue, 28 Jan 2020 22:53:08 -0500 DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.codeaurora.org; q=dns/txt; s=smtp; t=1580269986; h=In-Reply-To: Content-Type: MIME-Version: References: Message-ID: Subject: Cc: To: From: Date: Sender; bh=8Yfsu70kjW1KcreCoB/LC/iySnJRGXuVvHJxxjEM3Uk=; b=NqBj4uIhKgTEwAztJ9UWdKalB5/CwwqeTtsDE5k+9IwXX9tLQ6ao5dkzjZYXBOahWFMMgE64 HElDGtsrQ8xHcgn9rJIMOYZ9XaCTxRmhl4mhvrgAndkvmCOvpldxumKlezKSCmjlVS6fDrp7 hUZCH34T6GZomx2LzCLP92n4qlg= X-Mailgun-Sending-Ip: 104.130.122.26 X-Mailgun-Sid: WyI0MWYwYSIsICJsaW51eC1rZXJuZWxAdmdlci5rZXJuZWwub3JnIiwgImJlOWU0YSJd Received: from smtp.codeaurora.org (ec2-35-166-182-171.us-west-2.compute.amazonaws.com [35.166.182.171]) by mxa.mailgun.org with ESMTP id 5e3101a1.7f5f243f0650-smtp-out-n02; Wed, 29 Jan 2020 03:53:05 -0000 (UTC) Received: by smtp.codeaurora.org (Postfix, from userid 1001) id B3899C4479C; Wed, 29 Jan 2020 03:53:05 +0000 (UTC) Received: from codeaurora.org (blr-c-bdr-fw-01_GlobalNAT_AllZones-Outside.qualcomm.com [103.229.19.19]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: pkondeti) by smtp.codeaurora.org (Postfix) with ESMTPSA id 03B47C43383; Wed, 29 Jan 2020 03:53:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 03B47C43383 Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; spf=none smtp.mailfrom=pkondeti@codeaurora.org Date: Wed, 29 Jan 2020 09:22:58 +0530 From: Pavan Kondeti To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, qperret@google.com, adharmap@codeaurora.org Subject: Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan Message-ID: <20200129035258.GB27398@codeaurora.org> References: <20200126200934.18712-1-valentin.schneider@arm.com> <20200126200934.18712-2-valentin.schneider@arm.com> <20200128062245.GA27398@codeaurora.org> <1ed322d6-0325-ecac-cc68-326a14b8c1dd@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1ed322d6-0325-ecac-cc68-326a14b8c1dd@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 28, 2020 at 11:30:26AM +0000, Valentin Schneider wrote: > Hi Pavan, > > On 28/01/2020 06:22, Pavan Kondeti wrote: > > Hi Valentin, > > > > On Sun, Jan 26, 2020 at 08:09:32PM +0000, Valentin Schneider wrote: > >> > >> +static inline int check_cpu_capacity(struct rq *rq, struct sched_domain *sd); > >> + > >> +/* > >> + * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which > >> + * the task fits. If no CPU is big enough, but there are idle ones, try to > >> + * maximize capacity. > >> + */ > >> +static int select_idle_capacity(struct task_struct *p, int target) > >> +{ > >> + unsigned long best_cap = 0; > >> + struct sched_domain *sd; > >> + struct cpumask *cpus; > >> + int best_cpu = -1; > >> + struct rq *rq; > >> + int cpu; > >> + > >> + if (!static_branch_unlikely(&sched_asym_cpucapacity)) > >> + return -1; > >> + > >> + sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target)); > >> + if (!sd) > >> + return -1; > >> + > >> + sync_entity_load_avg(&p->se); > >> + > >> + cpus = this_cpu_cpumask_var_ptr(select_idle_mask); > >> + cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); > >> + > >> + for_each_cpu_wrap(cpu, cpus, target) { > >> + rq = cpu_rq(cpu); > >> + > >> + if (!available_idle_cpu(cpu)) > >> + continue; > >> + if (task_fits_capacity(p, rq->cpu_capacity)) > >> + return cpu; > > > > I have couple of questions. > > > > (1) Any particular reason for not checking sched_idle_cpu() as a backup > > for the case where all eligible CPUs are busy? select_idle_cpu() does > > that. > > > > No particular reason other than we didn't consider it, I think. I don't see > any harm in folding it in, I'll do that for v4. I am curious however; are > you folks making use of SCHED_IDLE? AFAIA Android isn't making use of it > yet, though Viresh paved the way for that to happen. > We are not using SCHED_IDLE in product setups. I am told Android may use it for background tasks in future. I am not completely sure though. I asked it because select_idle_cpu() is using it. > > (2) Assuming all CPUs are busy, we return -1 from here and end up > > calling select_idle_cpu(). The traversal in select_idle_cpu() may be > > waste in cases where sd_llc == sd_asym_cpucapacity . For example SDM845. > > Should we worry about this? > > > > Before v3, since we didn't have the fallback CPU selection within > select_idle_capacity(), we would need the fall-through to select_idle_cpu() > (we could've skipped an idle CPU just because its capacity wasn't high > enough). > > That's not the case anymore, so indeed we may be able to bail out of > select_idle_sibling() right after select_idle_capacity() (or after the > prev / recent_used_cpu checks). Our only requirement here is that sd_llc > remains a subset of sd_asym_cpucapacity. > > So far for Arm topologies we can have: > - sd_llc < sd_asym_cpucapacity (e.g. legacy big.LITTLE like Juno) > - sd_llc == sd_asym_cpucapacity (e.g. DynamIQ like SDM845) > > I'm slightly worried about sd_llc > sd_asym_cpucapacity ever being an > actual thing - I don't believe it makes much sense, but that's not stopping > anyone. > > AFAIA we (Arm) *currently* don't allow that with big.LITTLE or DynamIQ, nor > do I think it can happen with the default scheduler topology where MC is > the last possible level we can have as sd_llc. > > So it *might* be a safe assumption - and I can still add a SCHED_WARN_ON(). Agreed. -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.