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=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, 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 4548BC32771 for ; Fri, 24 Jan 2020 13:03:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D53A20838 for ; Fri, 24 Jan 2020 13:03:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389018AbgAXNDs (ORCPT ); Fri, 24 Jan 2020 08:03:48 -0500 Received: from foss.arm.com ([217.140.110.172]:51444 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388485AbgAXNDp (ORCPT ); Fri, 24 Jan 2020 08:03:45 -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 BDBD61007; Fri, 24 Jan 2020 05:03:44 -0800 (PST) Received: from e113632-lin.cambridge.arm.com (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 8A2E03F68E; Fri, 24 Jan 2020 05:03:43 -0800 (PST) From: Valentin Schneider To: linux-kernel@vger.kernel.org Cc: 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: [PATCH v2 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan Date: Fri, 24 Jan 2020 13:02:11 +0000 Message-Id: <20200124130213.24886-2-valentin.schneider@arm.com> X-Mailer: git-send-email 2.24.0 In-Reply-To: <20200124130213.24886-1-valentin.schneider@arm.com> References: <20200124130213.24886-1-valentin.schneider@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Morten Rasmussen On asymmetric CPU capacity topologies, we currently rely on wake_cap() to drive select_task_rq_fair() towards either - its slow-path (find_idlest_cpu()) if either the previous or current (waking) CPU has too little capacity for the waking task - its fast-path (select_idle_sibling()) otherwise Commit 3273163c6775 ("sched/fair: Let asymmetric CPU configurations balance at wake-up") points out that this relies on the assumption that "[...]the CPU capacities within an SD_SHARE_PKG_RESOURCES domain (sd_llc) are homogeneous". This assumption no longer holds on newer generations of big.LITTLE systems (DynamIQ), which can accommodate CPUs of different compute capacity within a single LLC domain. To hopefully paint a better picture, a regular big.LITTLE topology would look like this: +---------+ +---------+ | L2 | | L2 | +----+----+ +----+----+ |CPU0|CPU1| |CPU2|CPU3| +----+----+ +----+----+ ^^^ ^^^ LITTLEs bigs which would result in the following scheduler topology: DIE [ ] <- sd_asym_cpucapacity MC [ ] [ ] <- sd_llc 0 1 2 3 Conversely, a DynamIQ topology could look like: +-------------------+ | L3 | +----+----+----+----+ | L2 | L2 | L2 | L2 | +----+----+----+----+ |CPU0|CPU1|CPU2|CPU3| +----+----+----+----+ ^^^^^ ^^^^^ LITTLEs bigs which would result in the following scheduler topology: MC [ ] <- sd_llc, sd_asym_cpucapacity 0 1 2 3 What this means is that, on DynamIQ systems, we could pass the wake_cap() test (IOW presume the waking task fits on the CPU capacities of some LLC domain), thus go through select_idle_sibling(). This function operates on an LLC domain, which here spans both bigs and LITTLEs, so it could very well pick a CPU of too small capacity for the task, despite there being fitting idle CPUs - it very much depends on the CPU iteration order, on which we have absolutely no guarantees capacity-wise. Introduce yet another select_idle_sibling() helper function that takes CPU capacity into account. The policy is basically to pick the first idle CPU which is big enough for the task (task_util * margin < cpu_capacity). Unlike other select_idle_sibling() helpers, this one operates on the sd_asym_cpucapacity sched_domain pointer, which is guaranteed to span all known CPU capacities in the system. As such, this will work for both "legacy" big.LITTLE (LITTLEs & bigs split at MC, joined at DIE) and for newer DynamIQ systems (e.g. LITTLEs and bigs in the same MC domain). Co-authored-by: Valentin Schneider Signed-off-by: Morten Rasmussen Signed-off-by: Valentin Schneider --- kernel/sched/fair.c | 37 +++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fe4e0d7753756..c44b135f61ad0 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5894,6 +5894,38 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t return cpu; } +/* + * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which + * the task fits. + */ +static int select_idle_capacity(struct task_struct *p, int target) +{ + struct sched_domain *sd; + struct cpumask *cpus; + int cpu; + + if (!static_branch_unlikely(&sched_asym_cpucapacity)) + return -1; + + sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target)); + if (!sd) + return -1; + + 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) { + if (!available_idle_cpu(cpu)) + continue; + if (!task_fits_capacity(p, capacity_of(cpu))) + continue; + + return cpu; + } + + return -1; +} + /* * Try and locate an idle core/thread in the LLC cache domain. */ @@ -5902,6 +5934,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) struct sched_domain *sd; int i, recent_used_cpu; + /* For asymmetric capacities, try to be smart about the placement */ + i = select_idle_capacity(p, target); + if ((unsigned)i < nr_cpumask_bits) + return i; + if (available_idle_cpu(target) || sched_idle_cpu(target)) return target; -- 2.24.0