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 +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,
