[PATCH v3 05/20] sysfs: Add preferred CPU file

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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                                 |  8 ++++++++
 2 files changed, 19 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc8..354058c07d65 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -806,3 +806,14 @@ 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:		May 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 based on steal time. With CONFIG_PREFERRED_CPU=n it
+		is same as online CPUs. See sched-arch.rst for more details.
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 875abdc9942e..0c6647805805 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -391,6 +391,13 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
 }
 #endif
 
+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);
+
 const struct bus_type cpu_subsys = {
 	.name = "cpu",
 	.dev_name = "cpu",
@@ -532,6 +539,7 @@ static struct attribute *cpu_root_attrs[] = {
 #ifdef CONFIG_GENERIC_CPU_AUTOPROBE
 	&dev_attr_modalias.attr,
 #endif
+	&dev_attr_preferred.attr,
 	NULL
 };
 
-- 
2.47.3