[PATCH v2 0/1] tick/nohz: avoid showing '(null)' if nohz_full= not set

[PATCH v2 0/1] tick/nohz: avoid showing '(null)' if nohz_full= not set

[Aaron Tomlin]

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, when Linux kernel boot-time parameter "nohz_full=" is
correctly specified (see tick_nohz_full_setup() and 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.


Changes since v1 [1]:

 - Refactor to use an if statement (Greg KH)

[1]: https://lore.kernel.org/lkml/20251006005824.76187-1-atomlin@atomlin.com/

Aaron Tomlin (1):
  tick/nohz: avoid showing '(null)' if nohz_full= not set

 drivers/base/cpu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

-- 
2.49.0

[PATCH v2 1/1] tick/nohz: avoid showing '(null)' if nohz_full= not set

[Aaron Tomlin]

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.

Fixes: 6570a9a1ce3a1 ("show nohz_full cpus in sysfs")
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 drivers/base/cpu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index fa0a2eef93ac..10924fb5103b 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -304,7 +304,11 @@ 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));
+	if (!tick_nohz_full_mask)
+		return sysfs_emit(buf, "\n");
+	else
+		return sysfs_emit(buf, "%*pbl\n",
+				  cpumask_pr_args(tick_nohz_full_mask));
 }
 static DEVICE_ATTR(nohz_full, 0444, print_cpus_nohz_full, NULL);
 #endif
-- 
2.49.0

Re: [PATCH v2 1/1] tick/nohz: avoid showing '(null)' if nohz_full= not set

[Nathan Chancellor]

Hi Aaron,

On Mon, Oct 06, 2025 at 09:25:04PM -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.
> 
> Fixes: 6570a9a1ce3a1 ("show nohz_full cpus in sysfs")
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>  drivers/base/cpu.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index fa0a2eef93ac..10924fb5103b 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -304,7 +304,11 @@ 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));
> +	if (!tick_nohz_full_mask)

This should use cpumask_available() to avoid the warning reported at
https://lore.kernel.org/202510100304.IpfE7EKh-lkp@intel.com/.

> +		return sysfs_emit(buf, "\n");
> +	else
> +		return sysfs_emit(buf, "%*pbl\n",
> +				  cpumask_pr_args(tick_nohz_full_mask));

You don't need the else since you return in the if, which will make the
diff smaller overall.

>  }
>  static DEVICE_ATTR(nohz_full, 0444, print_cpus_nohz_full, NULL);
>  #endif
> -- 
> 2.49.0
>