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

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@atomlin.com>
Link: https://patch.msgid.link/20251011011830.6670-3-atomlin@atomlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index c792ec66462d85b7bad37582f9409355645c536a..e3cb47eae982288b4377d4d16dfc2012c6d1b6a6 100644 (file)
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -305,7 +305,10 @@ static ssize_t nohz_full_show(struct device *dev,
                                    struct device_attribute *attr,
                                    char *buf)
 {
-       return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(tick_nohz_full_mask));
+       if (cpumask_available(tick_nohz_full_mask))
+               return sysfs_emit(buf, "%*pbl\n",
+                                 cpumask_pr_args(tick_nohz_full_mask));
+       return sysfs_emit(buf, "\n");
 }
 static DEVICE_ATTR_RO(nohz_full);
 #endif