linux-kernel - Re: [PATCH] tick/nohz: Expose housekeeping CPUs in sysfs

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Message-ID: <2025100652-scolding-tractor-8745@gregkh>
Date: Mon, 6 Oct 2025 08:33:57 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: rafael@...nel.org, dakr@...nel.org, riel@...riel.com,
	frederic@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick/nohz: Expose housekeeping CPUs in sysfs

On Sun, Oct 05, 2025 at 08:58:53PM -0400, Aaron Tomlin wrote:
> Expose the current system-defined list of housekeeping CPUs in a new
> sysfs file: /sys/devices/system/cpu/housekeeping.

But you did not document that new file in Documentation/ABI so it's a
bit hard to review if this is really correct or not.

> This provides userspace performance tuning tools and resource managers
> with a canonical, reliable method to accurately identify the cores
> responsible for essential kernel maintenance workloads (RCU, timer
> callbacks, and unbound workqueues). Currently, tooling must manually
> calculate the housekeeping set by parsing complex kernel boot parameters
> (like isolcpus= and nohz_full=) and system topology, which is prone to
> error. This dedicated file simplifies the configuration of low-latency
> workloads.
> 
> Signed-off-by: Aaron Tomlin <atomlin@...mlin.com>
> ---
>  drivers/base/cpu.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index f694b4b24d6c..c6511658f6cd 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -300,6 +300,19 @@ static ssize_t print_cpus_isolated(struct device *dev,
>  }
>  static DEVICE_ATTR(isolated, 0444, print_cpus_isolated, NULL);
>  
> +static ssize_t print_cpus_hk(struct device *dev,
> +			     struct device_attribute *attr, char *buf)
> +{
> +	const struct cpumask *hk_mask;
> +
> +	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
> +
> +	return housekeeping_enabled(HK_TYPE_KERNEL_NOISE) ?
> +		sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(hk_mask)) :
> +		sysfs_emit(buf, "\n");

Please write out if statements, it makes it easier over time to
maintain.

> +static DEVICE_ATTR(housekeeping, 0444, print_cpus_hk, NULL);

DEVICE_ATTR_RO()?

thanks,

greg k-h