On 10/15/26 8:43 AM, Peng Wang wrote:
Expose nohz_full_allowed under /sys/devices/system/cpu/ as the subset of
nohz_full CPUs permitted to suppress their non-idle scheduler tick.
Writes replace the entire list; reads report policy, not current tick
state. Reject invalid input before updating policy. Document asynchronous
revocation, offline CPU configuration and persistence across hotplug.
The existing nohz_full attribute continues to report membership.
Co-developed-by: He Rongguang <[email protected]>
Signed-off-by: He Rongguang <[email protected]>
Signed-off-by: Peng Wang <[email protected]>
---
.../ABI/testing/sysfs-devices-system-cpu | 40 ++++++++++++++++
drivers/base/cpu.c | 46 +++++++++++++++++++
2 files changed, 86 insertions(+)
diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu
b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc89..edf3408fd84c8 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -767,6 +767,46 @@ Description: Preferred MTE tag checking mode
See also: Documentation/arch/arm64/memory-tagging-extension.rst
+What: /sys/devices/system/cpu/nohz_full_allowed
I do have a problem with the name of this sysfs file. nohz_full is more
than just the tick. The naming may introduce the wrong impression that
all nohz_full behavior can be controlled. May be a name like
"nohz_tick_allowed" which is much more specific will be better.
Regards,
Longman
+Date: October 2026
+Contact: Linux kernel mailing list <[email protected]>
+Description:
+ (RW) CPUs permitted to stop the scheduler tick while non-idle.
+ Available with CONFIG_NO_HZ_FULL. The default is the complete
+ nohz_full CPU list. Readback reports permission, not the current
+ tick state: all the usual tick dependencies still apply.
+
+ A write replaces the entire list, using CPU-list syntax (for
+ example, "1-3,5"). An empty line revokes permission from all
+ nohz_full CPUs. Invalid input is rejected without changing
+ policy. A parsed list containing CPUs outside nohz_full fails
+ with EINVAL. CPU numbers beyond the mask range and numeric
+ overflows are rejected by the CPU-list parser with ERANGE and
+ EOVERFLOW, respectively. Input must be shorter than PAGE_SIZE
+ bytes, otherwise E2BIG is returned. Embedded NULs and newlines
+ other than a trailing newline are rejected with EINVAL. Valid
+ writes without an active nohz_full configuration fail with
+ EOPNOTSUPP.
+
+ Reads fail with EOVERFLOW if the formatted output reaches
+ PAGE_SIZE - 1 bytes, rather than returning a potentially
+ truncated CPU list.
+
+ Revoking permission adds a per-CPU tick dependency. Online CPUs
+ are notified through the existing asynchronous kick mechanism
+ to restart a stopped non-idle tick. A successful write means
+ the policy is updated and any needed notification is queued;
+ it does not wait for tick restart. Changes to multiple CPUs
+ are not simultaneous. Granting permission only removes this
+ dependency and does not force the tick to stop.
+
+ Offline CPUs may be configured without sending notifications.
+ Policy persists across CPU offline/online. Idle tick suppression
+ is unchanged; the policy applies when a CPU leaves idle.
+
+ This interface does not change nohz_full membership, context
+ tracking, RCU offloading, or housekeeping configuration.
+
What: /sys/devices/system/cpu/nohz_full
Date: Apr 2015
Contact: Linux kernel mailing list <[email protected]>
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 69e52fed42415..299ce61cef895 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -324,6 +324,51 @@ static ssize_t nohz_full_show(struct device *dev,
return sysfs_emit(buf, "\n");
}
static DEVICE_ATTR_RO(nohz_full);
+
+static ssize_t nohz_full_allowed_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ cpumask_var_t mask;
+ ssize_t ret;
+
+ if (!alloc_cpumask_var(&mask, GFP_KERNEL))
+ return -ENOMEM;
+
+ tick_nohz_full_get_allowed(mask);
+ ret = sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(mask));
+
+ free_cpumask_var(mask);
+
+ /* Never present a truncated CPU list as the complete policy. */
+ return ret == PAGE_SIZE - 1 ? -EOVERFLOW : ret;
+}
+
+static ssize_t nohz_full_allowed_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ cpumask_var_t mask;
+ int ret;
+
+ /* kernfs may truncate an oversized write to PAGE_SIZE. */
+ if (count >= PAGE_SIZE)
+ return -E2BIG;
+ if (memchr(buf, '\0', count) ||
+ (count > 1 && memchr(buf, '\n', count - 1)))
+ return -EINVAL;
+
+ if (!alloc_cpumask_var(&mask, GFP_KERNEL))
+ return -ENOMEM;
+
+ ret = cpulist_parse(buf, mask);
+ if (!ret)
+ ret = tick_nohz_full_set_allowed(mask);
+
+ free_cpumask_var(mask);
+
+ return ret ? ret : count;
+}
+static DEVICE_ATTR_RW(nohz_full_allowed);
#endif
#ifdef CONFIG_CRASH_HOTPLUG
@@ -525,6 +570,7 @@ static struct attribute *cpu_root_attrs[] = {
&dev_attr_housekeeping.attr,
#ifdef CONFIG_NO_HZ_FULL
&dev_attr_nohz_full.attr,
+ &dev_attr_nohz_full_allowed.attr,
#endif
#ifdef CONFIG_CRASH_HOTPLUG
&dev_attr_crash_hotplug.attr,
