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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 241BAD44148 for ; Tue, 19 Nov 2024 12:29:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:MIME-Version:Date:Message-ID:From:References:To: Subject:CC:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=NzY5TtQm6Ue6u4zoUA9OEnl0DUpLjxzz4KUu1sghaI4=; b=X2qcFw1hrNkXK/DgPI67sieZHx kw/Zs+HzXWQAR43spxD9V8OOYyp1tNxyT9FBL3VtPxK3eBL8XDqA+RFveXg4jrJ/RazNeV0ds5BGN rAra977gO3nWgiip0n+YqY454Ckq+ML8USnfXxHCgkDRi0JOS/xrKj55K0PwKezG5xNvNmMlxt2B7 NKaStjyceRRDppSc0j0Kg/QsoyzQvV5sYjY92a0uZ8BIDXZbDlB9wRgXSvmcE2D+WBI7OeI6TeDyI ZBGDtz2Ma1ehgvsc4ipWH4mZUo7tTyaNWUkRuHKfmEcn30d1OdpKmClwGVht/Vu2abxmqDpPC2QVA QX3WFjhw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1tDNLa-0000000CMs1-3RBE; Tue, 19 Nov 2024 12:28:46 +0000 Received: from szxga04-in.huawei.com ([45.249.212.190]) by bombadil.infradead.org with esmtps (Exim 4.98 #2 (Red Hat Linux)) id 1tDNKd-0000000CMbe-29Ad for linux-arm-kernel@lists.infradead.org; Tue, 19 Nov 2024 12:27:49 +0000 Received: from mail.maildlp.com (unknown [172.19.162.112]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4Xt3d26qkHz21kWm; Tue, 19 Nov 2024 20:26:14 +0800 (CST) Received: from kwepemd200014.china.huawei.com (unknown [7.221.188.8]) by mail.maildlp.com (Postfix) with ESMTPS id 17D7014010C; Tue, 19 Nov 2024 20:27:36 +0800 (CST) Received: from [10.67.121.177] (10.67.121.177) by kwepemd200014.china.huawei.com (7.221.188.8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1258.34; Tue, 19 Nov 2024 20:27:35 +0800 CC: , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH v9 1/4] cpu/SMT: Provide a default topology_is_primary_thread() To: Dietmar Eggemann , Pierre Gondois References: <20241114141127.23232-1-yangyicong@huawei.com> <20241114141127.23232-2-yangyicong@huawei.com> <427bd639-33c3-47e4-9e83-68c428eb1a7d@arm.com> <3876e185-2fcc-25dd-e70d-93fdcbf8fc37@huawei.com> From: Yicong Yang Message-ID: <6d732b89-d3d4-7bd6-d60e-e8bea05d05cf@huawei.com> Date: Tue, 19 Nov 2024 20:27:34 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.121.177] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To kwepemd200014.china.huawei.com (7.221.188.8) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241119_042747_867389_5FA82AA4 X-CRM114-Status: GOOD ( 20.98 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 2024/11/18 23:04, Dietmar Eggemann wrote: > On 18/11/2024 11:50, Yicong Yang wrote: >> On 2024/11/15 17:42, Pierre Gondois wrote: >>> Hello Yicong, >>> >>> >>> On 11/14/24 15:11, Yicong Yang wrote: >>>> From: Yicong Yang > > [...] > >>>>   diff --git a/include/linux/topology.h b/include/linux/topology.h >>>> index 52f5850730b3..b8e860276518 100644 >>>> --- a/include/linux/topology.h >>>> +++ b/include/linux/topology.h >>>> @@ -240,6 +240,26 @@ static inline const struct cpumask *cpu_smt_mask(int cpu) >>>>   } >>>>   #endif >>>>   +#ifndef topology_is_primary_thread >>>> + >>>> +#define topology_is_primary_thread topology_is_primary_thread >>>> + >>>> +static inline bool topology_is_primary_thread(unsigned int cpu) >>>> +{ >>>> +    /* >>>> +     * On SMT hotplug the primary thread of the SMT won't be disabled. >>>> +     * Architectures do have a special primary thread (e.g. x86) need >>>> +     * to override this function. Otherwise just make the first thread >>>> +     * in the SMT as the primary thread. >>>> +     * >>>> +     * The sibling cpumask of an offline CPU contains always the CPU >>>> +     * itself. >>> >>> As Thomas suggested, would it be possible to check it for other >>> architectures ? >>> For instance for loongarch at arch/loongarch/kernel/smp.c, >>> clear_cpu_sibling_map() seems to completely clear &cpu_sibling_map[cpu] >>> when a CPU is put offline. This would make topology_sibling_cpumask(cpu) >>> to be empty and cpu_bootable() return false if the CPU never booted before. >>> >> >> cpu_bootable() only affects architectures select HOTPLUG_SMT, otherwise it'll always >> return true. Since x86 and powerpc have their own illustration of primary thread and >> have an override version of this funciton, arm64 is the only user now by this patchset. >> We have this guarantee for arm64 and also for other architectures using arch_topology.c >> (see clear_cpu_topology()). So if loogarch has a different implementation, they >> should implement a topology_is_primary_thread() variant to support HOTPLUG_SMT. > > I also stumbled over this sentence. > > drivers/base/arch_topology.c: > > void clear_cpu_topology(int cpu) (2) > > ... > cpumask_set_cpu(cpu, &cpu_topo->thread_sibling) (4) > > void __init reset_cpu_topology(void) (1) > > for_each_possible_cpu(cpu) > > ... > clear_cpu_topology(cpu) (2) > > #if defined(CONFIG_ARM64) || defined(CONFIG_RISCV) (3) > void __init init_cpu_topology(void) > > reset_cpu_topology() (1) > ... > > Does this mean the default implementation relies on (4), i.e. is only > valid for arm64 and riscv? (3) > Do all the other archs then have to overwrite the default implementation > (like x86 and powerpc) if they want to implement CONFIG_HOTPLUG_SMT? > I think yes if they have problems with the default implementation. That's what used to be to support HOTPLUG_SMT before this, each arthitecture provides their own variant of topology_is_primary_thread. The current approach may also work since cpu_bootable() will make all the CPUs boot at least once: static inline bool cpu_bootable(unsigned int cpu) { [...] if (topology_is_primary_thread(cpu)) return true; return !cpumask_test_cpu(cpu, &cpus_booted_once_mask); // True if not booted } The boot process will be like below. cpu_bootable() is checked twice: -> cpu_up() cpu_bootable() (1) [...] cpuhp_bringup_ap() [ archs usually update the sibling_mask in start_secondary() here ] bringup_wait_for_ap_online() if (!cpu_bootable(cpu)) (2) return -ECANCELED // roll back and offline this CPU So an empty mask in (1) won't block the CPU going online. And the default topology_is_primary_thread() should work if sibling mask updated before the checking in (2). I hacked x86 to use the default topology_is_primary_thread and tested on a qemu vm and it seems also work (just for test since the primary thread should not always be the 1st thread in the core on x86, not quite sure). Thanks.