From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout04.his.huawei.com (canpmsgout04.his.huawei.com [113.46.200.219]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 046401A4F2F for ; Sat, 25 Apr 2026 02:05:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.219 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777082741; cv=none; b=LOBLBoSPdNgc3S1quusEgfW2px1OtdkHX4m5MgQwDOg0TiZcWFBsOIvW7h+wclapPoiDePWuCJgqXDtn9EEiTU8oELZM8UIV27R7Vhbktg4PwhoGNgC2fsldDVAsltxJXlq1Uh/6w82jyd+jL0r9f7o9OOYVpW6v6QkdlrnMcUY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777082741; c=relaxed/simple; bh=cAJlRd/kwNC6RYpn1wnxlOSgKswdTw4OjTl9cweeTrM=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=m4bi5SA0TQY8DVw2vzC4m+iJ8IrQNuO4QD5+KD6RergRZoyplwKviKvYIQ8B2keZoL2LFAGMOVTLMNGQVX09ZGsYRRSnIpnuHDgcEyJJGfksukaqEakbaGdi9GtwGW1uF1C0k2Exf1/9RfLo26rSsbeQZo/EsZSYlL1AMCmIvpA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=o37jocVR; arc=none smtp.client-ip=113.46.200.219 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="o37jocVR" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=EAuCq9NQCBLH98ryzbAu/eAZd+F8DHa7KR+l++83vqc=; b=o37jocVRSIaDbfz67h+yPkFhoRW2o+XJXPTmsTIFgm2iNLBcwkmJTE8p+halAlztHW1ZAU0E7 V6LUk/wAYoAaMCxBlPnC9IpRH9UlLP/knAM5vVhvqU7WvQhXQtwQHfAKKN/6JCF3yQ4K67UnHSm Ol0t+dg+Z0uQfr7vudsaG3g= Received: from mail.maildlp.com (unknown [172.19.162.140]) by canpmsgout04.his.huawei.com (SkyGuard) with ESMTPS id 4g2Y0Q4RLSz1prLm; Sat, 25 Apr 2026 09:59:02 +0800 (CST) Received: from dggpemf500011.china.huawei.com (unknown [7.185.36.131]) by mail.maildlp.com (Postfix) with ESMTPS id A6BF72012A; Sat, 25 Apr 2026 10:05:29 +0800 (CST) Received: from [10.67.109.254] (10.67.109.254) by dggpemf500011.china.huawei.com (7.185.36.131) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Sat, 25 Apr 2026 10:05:28 +0800 Message-ID: Date: Sat, 25 Apr 2026 10:05:30 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable() To: Catalin Marinas CC: Thomas Gleixner , , , , , Jonathan Cameron , , James Morse , References: <20260417075534.3745793-1-ruanjinjie@huawei.com> <87ldee0z1w.ffs@tglx> <78515da3-03a1-4fdb-a606-3fea9f4cd20b@huawei.com> From: Jinjie Ruan In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems500001.china.huawei.com (7.221.188.70) To dggpemf500011.china.huawei.com (7.185.36.131) On 4/24/2026 8:51 PM, Catalin Marinas wrote: > (updating Jonathan's email address to match MAINTAINERS) > > On Fri, Apr 24, 2026 at 09:56:24AM +0800, Jinjie Ruan wrote: >> On 4/24/2026 4:11 AM, Catalin Marinas wrote: >>> On Thu, Apr 23, 2026 at 08:32:34PM +0800, Jinjie Ruan wrote: >>>> On 4/23/2026 6:08 PM, Thomas Gleixner wrote: >>>>> On Sat, Apr 18 2026 at 12:55, Catalin Marinas wrote: >>>>>> Another option would have been to avoid marking such CPUs present but I >>>>>> think this will break other things. Yet another option is to register >>>>>> all CPU devices even if they never come up (like maxcpus greater than >>>>>> actual CPUs). >>>>>> >>>>>> Opinions? It might be an arm64+ACPI-only thing. >>>>> >>>>> I think so. The proper thing to do is to apply sane limits: >>>>> >>>>> 1) The possible CPUs enumerated by firmware N_POSSIBLE_FW >>>>> >>>>> 2) The maxcpus limit on the command line N_MAXCPUS_CL >>>>> >>>>> So the actual possible CPUs evaluates to: >>>>> >>>>> num_possible = min(N_POSSIBLE_FW, N_MAXCPUS_CL, CONFIG_NR_CPUS); >>>>> >>>>> The evaluation of the firmware should not mark CPUs present which are >>>>> actually not. ACPI gives you that information. See: >>>>> >>>>> 5.2.12.14 GIC CPU Interface (GICC) Structure >>>>> >>>>> in the ACPI spec. That has two related bits: >>>>> >>>>> Enabled: >>>>> >>>>> If this bit is set, the processor is ready for use. If this bit is >>>>> clear and the Online Capable bit is set, the system supports enabling >>>>> this processor during OS runtime. If this bit is clear and the Online >>>>> Capable bit is also clear, this processor is un- usable, and the >>>>> operating system support will not attempt to use it. >>>>> >>>>> Online Capable: >>>>> >>>>> The information conveyed by this bit depends on the value of the >>>>> Enabled bit. If the Enabled bit is set, this bit is reserved and must >>>>> be zero. Otherwise, if this bit is set, the system supports enabling >>>>> this processor later during OS runtime >>>>> >>>>> So the combination of those gives you the right answer: >>>>> >>>>> Enabled Online >>>>> Capable >>>>> 0 0 Not present, not possible >>>>> 0 1 Not present, but possible to "hotplug" layter >>>>> 1 0 Present >>>>> 1 1 Invalid >>>> >>>> On x86, it seems that all CPUs with the ACPI_MADT_ENABLED bit set will >>>> be marked as present. >>>> >>>> acpi_parse_x2apic() >>>> -> enabled = processor->lapic_flags & ACPI_MADT_ENABLED >>>> -> topology_register_apic(enabled) >>>> -> topo_register_apic(enabled) >>>> -> set_cpu_present(cpu, true) >>> >>> Yes but arm64 marks all CPUs present even if !ACPI_MADT_ENABLED as we >>> don't have the notion of hardware CPU hotplug. >>> >>> I need to dig some more into the original vCPU hotplug support and why >>> we ended up with all CPUs marked as present even if not calling >>> register_cpu(): >>> >>> https://lore.kernel.org/linux-arm-kernel/20240529133446.28446-1-Jonathan.Cameron@huawei.com/ >>> >>> What's the MADT GICC provided by qemu with "-smp cpus=4,maxcpus=8"? If >>> it says Enabled for the first 4 and Online Capable for the rest, maybe >>> we can try something like below: >> >> Yes, you are absolutely right,Enabled for the first 4(with GIC Flags: >> 0x1, bit0 set) and Online Capable for the rest(with GIC Flags: 0x8, bit3 >> set). The ACPI MADT disassembly result is as follows: > > That's great, thanks for checking. > > I'd like to get some feedback from Jonathan and James as they > contributed the vCPU hotplug support. The reason was for kubernetes to > add vCPUs to an existing VM. Hopefully no-one relied on > /sys/devices/system/cpu/present to report 0-7 in the above > configuration. Yes, only cpu 0~3 present. # cat /sys/devices/system/cpu/possible 0-7 # cat /sys/devices/system/cpu/enabled 0-3 # cat /sys/devices/system/cpu/present 0-3 # cat /sys/devices/system/cpu/online 0-3 # cat /sys/devices/system/cpu/offline 4-7 > > Have you tried the vCPU hotplug with this patch (the original use-case)? Yes，the basic vCPU hotplug has no problem as below: Add the last four CPUs: (qemu) device_add host-arm-cpu,id=cpu4,core-id=2,thread-id=0 (qemu) [ 678.984281] ACPI: CPU4 has been hot-added (qemu) device_add host-arm-cpu,id=cpu5,core-id=2,thread-id=1 (qemu) [ 686.593536] ACPI: CPU5 has been hot-added (qemu) device_add host-arm-cpu,id=cpu6,core-id=3,thread-id=0 (qemu) [ 699.493530] ACPI: CPU6 has been hot-added (qemu) device_add host-arm-cpu,id=cpu7,core-id=3,thread-id=1 (qemu) [ 706.165770] ACPI: CPU7 has been hot-added # cat /sys/devices/system/cpu/cpu*/online 1 1 1 1 0 0 0 0 # echo 1 > /sys/devices/system/cpu/cpu4/online [ 868.423001] Detected PIPT I-cache on CPU4 [ 868.437103] GICv3: CPU4: found redistributor 4 region 0:0x0000000008120000 [ 868.437193] GICv3: CPU4: using allocated LPI pending table @0x0000000040120000 [ 868.437330] CPU4: Booted secondary processor 0x0000000004 [0x410fd082] # echo 1 > /sys/devices/system/cpu/cpu5/online [ 871.855694] Detected PIPT I-cache on CPU5 [ 871.869725] GICv3: CPU5: found redistributor 5 region 0:0x0000000008140000 [ 871.869811] GICv3: CPU5: using allocated LPI pending table @0x0000000040130000 [ 871.869938] CPU5: Booted secondary processor 0x0000000005 [0x410fd082] # echo 1 > /sys/devices/system/cpu/cpu6/online [ 874.756497] Detected PIPT I-cache on CPU6 [ 874.770521] GICv3: CPU6: found redistributor 6 region 0:0x0000000008160000 [ 874.770606] GICv3: CPU6: using allocated LPI pending table @0x0000000040140000 [ 874.770748] CPU6: Booted secondary processor 0x0000000006 [0x410fd082] # echo 1 > /sys/devices/system/cpu/cpu7/online [ 878.212505] Detected PIPT I-cache on CPU7 [ 878.226560] GICv3: CPU7: found redistributor 7 region 0:0x0000000008180000 [ 878.226646] GICv3: CPU7: using allocated LPI pending table @0x0000000040150000 [ 878.226783] CPU7: Booted secondary processor 0x0000000007 [0x410fd082] # cat /sys/devices/system/cpu/cpu*/online 1 1 1 1 1 1 1 1 # cat /sys/devices/system/cpu/enabled 0-7 # cat /sys/devices/system/cpu/present 0-7 # cat /sys/devices/system/cpu/online 0-7 Unplug the last four CPUs. # (qemu) device_del cpu4 (qemu) [ 977.336251] psci: CPU4 killed (polled 0 ms) (qemu) device_del cpu5 (qemu) [ 979.948212] psci: CPU5 killed (polled 0 ms) (qemu) device_del cpu6 (qemu) [ 981.976337] psci: CPU6 killed (polled 0 ms) (qemu) device_del cpu7 (qemu) [ 984.476253] psci: CPU7 killed (polled 0 ms) # cat /sys/devices/system/cpu/cpu*/online 1 1 1 1 # cat /sys/devices/system/cpu/possible 0-7 # cat /sys/devices/system/cpu/enabled 0-3 # cat /sys/devices/system/cpu/present 0-3 # cat /sys/devices/system/cpu/online 0-3 # cat /sys/devices/system/cpu/offline 4-7 > > Anyway, feel free to post a v2 with the above proposal, cc Jonathan (on > kernel.org) and James Morse and we'll take it from there. You can add a > suggested-by me. > > Thanks. >