linux-kernel - Re: [PATCH 38/39] cpumask: Add enabled cpumask for present CPUs that can be brought online

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Date:   Tue, 24 Oct 2023 18:02:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 38/39] cpumask: Add enabled cpumask for present CPUs that
 can be brought online

On Tue, Oct 24, 2023 at 04:19:24PM +0100, Russell King wrote:
> From: James Morse <james.morse@....com>
> 
> The 'offline' file in sysfs shows all offline CPUs, including those
> that aren't present. User-space is expected to remove not-present CPUs
> from this list to learn which CPUs could be brought online.
> 
> CPUs can be present but not-enabled. These CPUs can't be brought online
> until the firmware policy changes, which comes with an ACPI notification
> that will register the CPUs.
> 
> With only the offline and present files, user-space is unable to
> determine which CPUs it can try to bring online. Add a new CPU mask
> that shows this based on all the registered CPUs.
> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
>  drivers/base/cpu.c      | 10 ++++++++++
>  include/linux/cpumask.h | 25 +++++++++++++++++++++++++
>  kernel/cpu.c            |  3 +++
>  3 files changed, 38 insertions(+)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 2b9cb2667654..f8bf1d4c7d71 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -95,6 +95,7 @@ void unregister_cpu(struct cpu *cpu)
>  {
>  	int logical_cpu = cpu->dev.id;
>  
> +	set_cpu_enabled(logical_cpu, false);
>  	unregister_cpu_under_node(logical_cpu, cpu_to_node(logical_cpu));
>  
>  	device_unregister(&cpu->dev);
> @@ -273,6 +274,13 @@ static ssize_t print_cpus_offline(struct device *dev,
>  }
>  static DEVICE_ATTR(offline, 0444, print_cpus_offline, NULL);
>  
> +static ssize_t print_cpus_enabled(struct device *dev,
> +				  struct device_attribute *attr, char *buf)
> +{
> +	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_enabled_mask));
> +}
> +static DEVICE_ATTR(enabled, 0444, print_cpus_enabled, NULL);

This needs to be documented somewhere in Documentation/ABI/ did I miss
that patch?

thanks,

greg k-h