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

On Fri, Oct 16, 2026 at 06:25:26PM +0200, Greg Kroah-Hartman wrote:

On Thu, Oct 15, 2026 at 08:43:09PM +0800, Peng Wang wrote:

quoted

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 <redacted>
Signed-off-by: He Rongguang <redacted>
Signed-off-by: Peng Wang <redacted>
---
 .../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 <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;

Why not?  What will userspace do then if it gets -EOVERFLOW as it can
not get the full list?

Hi Greg,

There is no way to retrieve the full list through this attribute.
The intent was to avoid returning a truncated list that userspace
could write back, unintentionally revoking permission for the omitted
CPUs. Returning an error does not solve that interface limitation.

quoted

+}
+
+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;

How are you getting a larger than PAGE_SIZE value here?

I cannot; kernfs caps count at PAGE_SIZE. The check is intended for
count == PAGE_SIZE, to avoid applying a potentially truncated prefix
as a complete list. It also rejects an exactly page-sized input.

quoted

+	if (memchr(buf, '\0', count) ||
+	    (count > 1 && memchr(buf, '\n', count - 1)))
+		return -EINVAL;

What are you testing for here, sorry, I can't easily figure it out.

The checks reject user-supplied NULs and non-final newlines.
For example, cpulist_parse() accepts "1\n2\n" as CPU 1 only, while
store() would report the full input as consumed. The NUL appended
by kernfs is at buf[count], outside the checked range.

I propose replacing the global attribute with
/sys/devices/system/cpu/cpuN/nohz_tick_allowed, available only on
nohz_full CPUs. Writing 0 would prevent non-idle tick suppression,
using the existing asynchronous kick when needed. Writing 1 would
allow suppression, subject to the existing tick dependencies.
Readback would report policy, not the current tick state.

Each write would affect only that CPU, without changing nohz_full
membership or other isolation settings. This would give up the
single-write whole-list replacement. Would that be a better fit?

Thanks,
Peng

thanks,

greg k-h