Re: [RFC PATCH 2/2] cpu: Add sysfs control of full dynticks policy

[Waiman Long <longman@redhat.com>]

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 <herongguang@linux.alibaba.com>
> Signed-off-by: He Rongguang <herongguang@linux.alibaba.com>
> Signed-off-by: Peng Wang <peng_wang@linux.alibaba.com>
> ---
>   .../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 <linux-kernel@vger.kernel.org>
> +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 <linux-kernel@vger.kernel.org>
> 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,