[PATCH v13 05/13] sysfs: Add preferred CPU file

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

Add a "preferred" file in /sys/devices/system/cpu/ when kernel is
built with CONFIG_PREFERRED_CPU=y.

This would help
- Users to quickly check which CPUs are marked as preferred.
- Userspace daemons such as irqbalance to use this mask to
  route irqs 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 <sshegde@linux.ibm.com>
---
 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..8f250b8693b8 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 <linux-kernel@vger.kernel.org>
 Description:
 		(RO) the list of CPUs that can be brought online.
+
+What:		/sys/devices/system/cpu/preferred
+Date:		Sep 2026
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+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-paravirt.rst for more details.
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 69e52fed4241..747915ff974f 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
 };
-- 
2.52.0