linux-kernel - Re: [PATCH RFC 01/22] arch_topology: Make register_cpu_capacity

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Message-ID: <20231128143722.000032db@Huawei.com>
Date:   Tue, 28 Nov 2023 14:37:22 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Russell King <rmk+kernel@...linux.org.uk>
CC:     <linux-pm@...r.kernel.org>, <loongarch@...ts.linux.dev>,
        <linux-acpi@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-riscv@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
        <x86@...nel.org>, <linux-csky@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-ia64@...r.kernel.org>,
        <linux-parisc@...r.kernel.org>,
        Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        <jianyong.wu@....com>, <justin.he@....com>,
        James Morse <james.morse@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH RFC 01/22] arch_topology: Make
 register_cpu_capacity_sysctl() tolerant to late CPUs

On Tue, 07 Nov 2023 10:29:23 +0000
Russell King <rmk+kernel@...linux.org.uk> wrote:

> From: James Morse <james.morse@....com>
> 
> 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@....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'm not keen on squirting sysfs files in from code so
might be nice to do that anyway and use is_visible() / sysfs_update_group()
but that would be a job for another day if at all.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>