[PATCH v2 04/17] sysfs: Add preferred CPU file

Shrikanth Hegde
2026-04-07T19:21:20Z

Add "preferred" file in /sys/devices/system/cpu

This offers
- User can quickly check which CPUs are marked as preferred at this
moment.
- Userspace algorithms irqbalance could use this mask to send irq into
preferred CPUs.

For example:
cat /sys/devices/system/cpu/online
0-719
cat /sys/devices/system/cpu/preferred
0-599 <<< Implies 0-599 are preferred for workloads and 600-719
should be avoided at this moment.

cat /sys/devices/system/cpu/preferred
0-719 <<< All CPUs are usable. There is no preferrence.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
Documentation/ABI/testing/sysfs-devices-system-cpu | 11 +++++++++++
drivers/base/cpu.c | 12 ++++++++++++
2 files changed, 23 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 3a05604c21bf..ffa05605923b 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -788,3 +788,14 @@ Date: Nov 2022
Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org>
Description:
(RO) the list of CPUs that can be brought online.
+
+What: /sys/devices/system/cpu/preferred
+Date: Apr 2026
+Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+ (RO) the list of preferred CPUs at this moment.
+ These are the only CPUs meant to be used at the moment.
+ Using CPU outside of the list could lead to more
+ contention of underlying physical CPU resource. Dynamically
+ changes to reflect the current situation by using
+ STEAL_MONITOR scheduler feature. Expects CONFIG_PARAVIRT=y
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 875abdc9942e..0a6cf37f2001 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -391,6 +391,15 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
}
#endif

+#ifdef CONFIG_PARAVIRT
+static ssize_t preferred_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_preferred_mask));
+}
+static DEVICE_ATTR_RO(preferred);
+#endif
+
const struct bus_type cpu_subsys = {
.name = "cpu",
.dev_name = "cpu",
@@ -531,6 +540,9 @@ static struct attribute *cpu_root_attrs[] = {
#endif
#ifdef CONFIG_GENERIC_CPU_AUTOPROBE
&dev_attr_modalias.attr,
+#endif
+#ifdef CONFIG_PARAVIRT
+ &dev_attr_preferred.attr,
#endif
NULL
};
--
2.47.3