[PATCH v3 0/2] tick/nohz: avoid showing '(null)' if nohz_full= not set

[PATCH v3 0/2] tick/nohz: avoid showing '(null)' if nohz_full= not set

[Aaron Tomlin]

Hi Nathan, Greg,

Thank you for your feedback so far.


Changes since v2 [1]

 - Removed unnecessary else statement (Nathan Chancellor)
 - Used cpumask_available() instead (Nathan Chancellor)
 - Used DEVICE_ATTR_RO for nohz_full attribute

Changes since v1 [2]

 - Refactor to use an if statement (Greg KH)

[1]: https://lore.kernel.org/lkml/20251007012504.16118-2-atomlin@atomlin.com/
[2]: https://lore.kernel.org/lkml/20251007012504.16118-1-atomlin@atomlin.com/

Aaron Tomlin (2):
  sysfs/cpu: Use DEVICE_ATTR_RO for nohz_full attribute
  tick/nohz: avoid showing '(null)' if nohz_full= not set

 drivers/base/cpu.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

-- 
2.49.0

[PATCH v3 1/2] sysfs/cpu: Use DEVICE_ATTR_RO for nohz_full attribute

[Aaron Tomlin]

The /sys/devices/system/cpu/nohz_full file is a read-only attribute that
reports the CPUs configured for tickless operation (CONFIG_NO_HZ_FULL=y).

The current definition uses the generic DEVICE_ATTR macro, which
unnecessarily requires specifying the file mode (0444) and a NULL
store operation pointer.

This patch converts the definition to use the dedicated DEVICE_ATTR_RO
macro. This correctly expresses the read-only nature of the attribute,
removes the redundant mode field, and simplifies the code. As a related
cleanup, rename the show function from print_cpus_nohz_full() to the
standard nohz_full_show() for consistency with common sysfs attribute
naming conventions.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 drivers/base/cpu.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index fa0a2eef93ac..c792ec66462d 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -301,12 +301,13 @@ static ssize_t print_cpus_isolated(struct device *dev,
 static DEVICE_ATTR(isolated, 0444, print_cpus_isolated, NULL);
 
 #ifdef CONFIG_NO_HZ_FULL
-static ssize_t print_cpus_nohz_full(struct device *dev,
-				    struct device_attribute *attr, char *buf)
+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));
 }
-static DEVICE_ATTR(nohz_full, 0444, print_cpus_nohz_full, NULL);
+static DEVICE_ATTR_RO(nohz_full);
 #endif
 
 #ifdef CONFIG_CRASH_HOTPLUG
-- 
2.49.0

[PATCH v3 2/2] 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.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 drivers/base/cpu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index c792ec66462d..e3cb47eae982 100644
--- 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
-- 
2.49.0