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Date:   Thu, 14 Sep 2023 13:01:26 +0100
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     James Morse <james.morse@arm.com>
CC:     <linux-pm@vger.kernel.org>, <loongarch@lists.linux.dev>,
        <linux-acpi@vger.kernel.org>, <linux-arch@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
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        <linux-riscv@lists.infradead.org>, <kvmarm@lists.linux.dev>,
        <x86@kernel.org>, Salil Mehta <salil.mehta@huawei.com>,
        Russell King <linux@armlinux.org.uk>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        <jianyong.wu@arm.com>, <justin.he@arm.com>,
        <gregkh@linuxfoundation.org>
Subject: Re: [RFC PATCH v2 11/35] arch_topology: Make
 register_cpu_capacity_sysctl() tolerant to late CPUs
Message-ID: <20230914130126.000069db@Huawei.com>
In-Reply-To: <20230913163823.7880-12-james.morse@arm.com>
References: <20230913163823.7880-1-james.morse@arm.com>
        <20230913163823.7880-12-james.morse@arm.com>
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On Wed, 13 Sep 2023 16:37:59 +0000
James Morse <james.morse@arm.com> wrote:

> register_cpu_capacity_sysctl() adds a property to sysfs that describes
> the CPUs capacity. This is done from a subsys_initcall() that assumes
> all possible CPUs are registered.
> 
> With CPU hotplug, possible CPUs aren't registered until they become
> present, (or for arm64 enabled). This leads to messages during boot:
> | register_cpu_capacity_sysctl: too early to get CPU1 device!
> and once these CPUs are added to the system, the file is missing.
> 
> Move this to a cpuhp callback, so that the file is created once
> CPUs are brought online. This covers CPUs that are added late by
> mechanisms like hotplug.
> One observable difference is the file is now missing for offline CPUs.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> If the offline CPUs thing is a problem for the tools that consume
> this value, we'd need to move cpu_capacity to be part of cpu.c's
> common_cpu_attr_groups.

I think we should do that anyway and then use an is_visible() if we want to
change whether it is visible in offline cpus.

Dynamic sysfs file creation is horrible - particularly when done
from an totally different file from where the rest of the attributes
are registered.  I'm curious what the history behind that is.

Whilst here, why is there a common_cpu_attr_groups which is
identical to the hotpluggable_cpu_attr_groups in base/cpu.c?


+CC GregKH
Given changes in drivers/base/

> ---
>  drivers/base/arch_topology.c | 38 ++++++++++++++++++++++++------------
>  1 file changed, 26 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index b741b5ba82bd..9ccb7daee78e 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -220,20 +220,34 @@ static DECLARE_WORK(update_topology_flags_work, update_topology_flags_workfn);
>  
>  static DEVICE_ATTR_RO(cpu_capacity);
>  
> +static int cpu_capacity_sysctl_add(unsigned int cpu)
> +{
> +	struct device *cpu_dev = get_cpu_device(cpu);
> +
> +	if (!cpu_dev)
> +		return -ENOENT;
> +
> +	device_create_file(cpu_dev, &dev_attr_cpu_capacity);
> +
> +	return 0;
> +}
> +
> +static int cpu_capacity_sysctl_remove(unsigned int cpu)
> +{
> +	struct device *cpu_dev = get_cpu_device(cpu);
> +
> +	if (!cpu_dev)
> +		return -ENOENT;
> +
> +	device_remove_file(cpu_dev, &dev_attr_cpu_capacity);
> +
> +	return 0;
> +}
> +
>  static int register_cpu_capacity_sysctl(void)
>  {
> -	int i;
> -	struct device *cpu;
> -
> -	for_each_possible_cpu(i) {
> -		cpu = get_cpu_device(i);
> -		if (!cpu) {
> -			pr_err("%s: too early to get CPU%d device!\n",
> -			       __func__, i);
> -			continue;
> -		}
> -		device_create_file(cpu, &dev_attr_cpu_capacity);
> -	}
> +	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "topology/cpu-capacity",
> +			  cpu_capacity_sysctl_add, cpu_capacity_sysctl_remove);
>  
>  	return 0;
>  }