Commit cd22926a authored by Aaron Tomlin's avatar Aaron Tomlin Committed by Greg Kroah-Hartman
Browse files

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: default avatarAaron Tomlin <atomlin@atomlin.com>
Link: https://patch.msgid.link/20251011012853.7539-2-atomlin@atomlin.com


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent f10c23fa

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

+11 −0
Original line number Diff line number Diff line
@@ -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

+14 −0
Original line number Diff line number Diff line
@@ -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