Add "preferred" file in /sys/devices/system/cpu
This would help
- Users to quickly check which CPUs are marked as preferred.
- Userspace daemons such as irqbalance to 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 preference.
Signed-off-by: Shrikanth Hegde <[email protected]>
---
Documentation/ABI/testing/sysfs-devices-system-cpu | 14 ++++++++++++++
drivers/base/cpu.c | 12 ++++++++++++
2 files changed, 26 insertions(+)
diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu
b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc8..676412c46b2a 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -806,3 +806,17 @@ Date: Nov 2022
Contact: Linux kernel mailing list <[email protected]>
Description:
(RO) the list of CPUs that can be brought online.
+
+What: /sys/devices/system/cpu/preferred
+Date: July 2026
+Contact: Linux kernel mailing list <[email protected]>
+Description:
+ (RO) the list of preferred CPUs applicable in
+ paravirtualized environments.
+
+ The steal governor driver dynamically adjusts this mask
+ based on observed steal time. Scheduling tasks on
+ CPUs outside of this list may lead to performance
+ degradations due to underlying physical CPU contention.
+
+ See Documentation/scheduler/sched-arch.rst for more details.
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 19d288a3c80c..5da2a96fb37e 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_PREFERRED_CPU
+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_PREFERRED_CPU
+ &dev_attr_preferred.attr,
#endif
NULL
};
--
2.47.3
