tick/nohz: Expose housekeeping CPUs in sysfs

Expose the current system-defined list of housekeeping CPUs in a new
sysfs file: /sys/devices/system/cpu/housekeeping.

This provides userspace performance tuning tools and resource managers
with a canonical, reliable method to accurately identify the cores
responsible for essential kernel maintenance workloads (RCU, timer
callbacks, and unbound workqueues). Currently, tooling must manually
calculate the housekeeping set by parsing complex kernel boot parameters
(like isolcpus= and nohz_full=) and system topology, which is prone to
error. This dedicated file simplifies the configuration of low-latency
workloads.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Link: https://patch.msgid.link/20251011012853.7539-2-atomlin@atomlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: aarontomlin

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -764,6 +764,17 @@ Description:
 		participate in load balancing. These CPUs are set by
 		boot parameter "isolcpus=".
 
+What:		/sys/devices/system/cpu/housekeeping
+Date:		Oct 2025
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of logical CPUs that are designated by the kernel as
+		"housekeeping". Each CPU are responsible for handling essential
+		system-wide background tasks, including RCU callbacks, delayed
+		timer callbacks, and unbound workqueues, minimizing scheduling
+		jitter on low-latency, isolated CPUs. These CPUs are set when boot
+		parameter "isolcpus=nohz" or "nohz_full=" is specified.
+
 What:		/sys/devices/system/cpu/crash_hotplug
 Date:		Aug 2023
 Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>

drivers/base/cpu.c

@@ -300,6 +300,19 @@ static ssize_t print_cpus_isolated(struct device *dev,
 }
 static DEVICE_ATTR(isolated, 0444, print_cpus_isolated, NULL);
 
+static ssize_t housekeeping_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	const struct cpumask *hk_mask;
+
+	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
+
+	if (housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
+		return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(hk_mask));
+	return sysfs_emit(buf, "\n");
+}
+static DEVICE_ATTR_RO(housekeeping);
+
 #ifdef CONFIG_NO_HZ_FULL
 static ssize_t nohz_full_show(struct device *dev,
 				    struct device_attribute *attr,
@@ -509,6 +522,7 @@ static struct attribute *cpu_root_attrs[] = {
 	&dev_attr_offline.attr,
 	&dev_attr_enabled.attr,
 	&dev_attr_isolated.attr,
+	&dev_attr_housekeeping.attr,
 #ifdef CONFIG_NO_HZ_FULL
 	&dev_attr_nohz_full.attr,
 #endif