linux-kernel - Re: [PATCH] tick/nohz: avoid showing '(null)' if nohz

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Message-ID: <2025100612-disarray-hazily-a49b@gregkh>
Date: Mon, 6 Oct 2025 08:34:27 +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: avoid showing '(null)' if nohz_full= not set

On Sun, Oct 05, 2025 at 08:58:24PM -0400, Aaron Tomlin wrote:
> In the context of CONFIG_NO_HZ_FULL=y, tick_nohz_full_mask (of type
> cpumask_var_t) is initialised to 0. Memory is only allocated to the cpumask
> data structure, in tick_nohz_full_setup(), when Linux kernel boot-time
> parameter "nohz_full=" is correctly specified (see housekeeping_setup()).
> If "nohz_full=" is not set and an attempt is made to read
> /sys/devices/system/cpu/nohz_full, '(null)' can be displayed:
> 
>     ❯ cat /sys/devices/system/cpu/nohz_full
>     (null)
> 
> This patch changes the output to print a newline (or 0x0A) instead of
> '(null)', making it consistent with print_cpus_isolated() behaviour.
> 
> Signed-off-by: Aaron Tomlin <atomlin@...mlin.com>
> ---
>  drivers/base/cpu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index fa0a2eef93ac..f694b4b24d6c 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -304,7 +304,9 @@ static DEVICE_ATTR(isolated, 0444, print_cpus_isolated, NULL);
>  static ssize_t print_cpus_nohz_full(struct device *dev,
>  				    struct device_attribute *attr, char *buf)
>  {
> -	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(tick_nohz_full_mask));
> +	return !tick_nohz_full_mask ?
> +		sysfs_emit(buf, "\n") :
> +		sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(tick_nohz_full_mask));

Please write out the if statement please.

thanks,

greg k-h